GPU correctness fixes and optimizations

draw/backdrop.odin

@@ -684,7 +684,13 @@ upload_backdrop_primitives :: proc(device: ^sdl.GPUDevice, pass: ^sdl.GPUCopyPas
 		sdl.GPUBufferUsageFlags{.GRAPHICS_STORAGE_READ},
 	)
 
-	prim_array := sdl.MapGPUTransferBuffer(device, GLOB.backdrop.primitive_buffer.transfer, false)
+	// cycle=true: this is a persistent per-frame streaming transfer buffer. The previous
+	// frame's UploadToGPUBuffer is almost certainly still in flight when we map here
+	// (allowedFramesInFlight defaults to 2 on Metal). Without cycling, the CPU memcpy below
+	// races the GPU's blit read on the same MTLBuffer.contents. Cycling rebinds the
+	// container's active internal buffer to an unbound one (or allocates a new one) — O(1)
+	// in steady state, no fence wait. See SDL_gpu_metal.m's METAL_INTERNAL_PrepareBufferForWrite.
+	prim_array := sdl.MapGPUTransferBuffer(device, GLOB.backdrop.primitive_buffer.transfer, true)
 	if prim_array == nil {
 		log.panicf("Failed to map backdrop primitive transfer buffer: %s", sdl.GetError())
 	}

draw/core_2d.odin

@@ -538,7 +538,9 @@ upload :: proc(device: ^sdl.GPUDevice, pass: ^sdl.GPUCopyPass) {
 			sdl.GPUBufferUsageFlags{.VERTEX},
 		)
 
-		vert_array := sdl.MapGPUTransferBuffer(device, GLOB.core_2d.vertex_buffer.transfer, false)
+		// cycle=true: see backdrop.odin upload_backdrop_primitives. Persistent per-frame
+		// streaming buffer; previous frame's blit is still in flight at map time.
+		vert_array := sdl.MapGPUTransferBuffer(device, GLOB.core_2d.vertex_buffer.transfer, true)
 		if vert_array == nil {
 			log.panicf("Failed to map vertex transfer buffer: %s", sdl.GetError())
 		}
@@ -569,7 +571,8 @@ upload :: proc(device: ^sdl.GPUDevice, pass: ^sdl.GPUCopyPass) {
 
 		grow_buffer_if_needed(device, &GLOB.core_2d.index_buffer, index_size, sdl.GPUBufferUsageFlags{.INDEX})
 
-		idx_array := sdl.MapGPUTransferBuffer(device, GLOB.core_2d.index_buffer.transfer, false)
+		// cycle=true: see vertex_buffer above.
+		idx_array := sdl.MapGPUTransferBuffer(device, GLOB.core_2d.index_buffer.transfer, true)
 		if idx_array == nil {
 			log.panicf("Failed to map index transfer buffer: %s", sdl.GetError())
 		}
@@ -596,7 +599,8 @@ upload :: proc(device: ^sdl.GPUDevice, pass: ^sdl.GPUCopyPass) {
 			sdl.GPUBufferUsageFlags{.GRAPHICS_STORAGE_READ},
 		)
 
-		prim_array := sdl.MapGPUTransferBuffer(device, GLOB.core_2d.primitive_buffer.transfer, false)
+		// cycle=true: see vertex_buffer above.
+		prim_array := sdl.MapGPUTransferBuffer(device, GLOB.core_2d.primitive_buffer.transfer, true)
 		if prim_array == nil {
 			log.panicf("Failed to map primitive transfer buffer: %s", sdl.GetError())
 		}

draw/examples/hellope.odin

@@ -279,12 +279,14 @@ hellope_custom :: proc() {
 	}
 
 	gauge := Gauge {
-		value = 0.73,
+		value    = 0.73,
-		color = {50, 200, 100, 255},
+		color    = {50, 200, 100, 255},
+		bg_color = {80, 80, 80, 255},
 	}
 	gauge2 := Gauge {
-		value = 0.45,
+		value    = 0.45,
-		color = {200, 100, 50, 255},
+		color    = {200, 100, 50, 255},
+		bg_color = {80, 80, 80, 255},
 	}
 
 	// `clay.CustomElementConfig.customData` is a rawptr; the Clay integration in `draw`
@@ -342,11 +344,11 @@ hellope_custom :: proc() {
 			// reflection inside the strip), and gauge2 is deferred-replayed by
 			// `prepare_clay_batch` after the bracket closes (renders crisp on top of the
 			// bracket output — unrelated to the strip since they don't overlap).
+			// `backgroundColor` is omitted on the gauges; bg lives on `Gauge.bg_color`. See `draw_custom`.
 			if clay.UI(clay.ID("gauge"))(
 			{
 				layout = {sizing = {clay.SizingFixed(300), clay.SizingFixed(30)}},
 				custom = {customData = &gauge_custom},
-				backgroundColor = {80, 80, 80, 255},
 			},
 			) {
 				if clay.UI(clay.ID("backdrop"))(
@@ -362,7 +364,6 @@ hellope_custom :: proc() {
 			{
 				layout = {sizing = {clay.SizingFixed(300), clay.SizingFixed(30)}},
 				custom = {customData = &gauge2_custom},
-				backgroundColor = {80, 80, 80, 255},
 			},
 			) {}
 		}
@@ -376,8 +377,9 @@ hellope_custom :: proc() {
 	}
 
 	Gauge :: struct {
-		value: f32,
+		value:    f32,
-		color: draw.Color,
+		color:    draw.Color,
+		bg_color: draw.Color,
 	}
 
 	draw_custom :: proc(layer: ^draw.Layer, bounds: draw.Rectangle, render_data: clay.CustomRenderData) {
@@ -386,14 +388,12 @@ hellope_custom :: proc() {
 		// before the union refactor.
 		gauge := cast(^Gauge)render_data.customData
 
+		// `gauge.bg_color` instead of `render_data.backgroundColor`: under Clay master, an
+		// element with both `custom.customData` and `backgroundColor` emits a Custom AND a
+		// Rectangle for the same bounds, in that order — the Rectangle paints over the
+		// callback's output. Carrying bg on user data sidesteps it.
 		border_width: f32 = 2
-		draw.rectangle(
+		draw.rectangle(layer, bounds, gauge.bg_color, outline_color = draw.WHITE, outline_width = border_width)
-			layer,
-			bounds,
-			draw.color_from_clay(render_data.backgroundColor),
-			outline_color = draw.WHITE,
-			outline_width = border_width,
-		)
 
 		fill := draw.Rectangle {
 			x      = bounds.x,