gpu: [compute] always bind valid materials texture

Currently, we run kernel4.comp with whatever texture was
bound (or none) when there are no materials in the set of layers.

However, Vulkan require every image binding to a compute shader to be
non-null and valid. This change works around that limitation by binding
a small dummy texture when no materials are needed.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/compute.go

@@ -73,6 +73,8 @@ type compute struct {
 
 		layerVertices []layerVertex
 		descriptors   *piet.Kernel4DescriptorSetLayout
+
+		nullMaterials driver.Texture
 	}
 	// imgAllocs maps imageOpData.handles to allocs.
 	imgAllocs map[interface{}]*atlasAlloc
@@ -409,6 +411,12 @@ func newCompute(ctx driver.Device) (*compute, error) {
 		conf:          new(config),
 		memHeader:     new(memoryHeader),
 	}
+	null, err := ctx.NewTexture(driver.TextureFormatRGBA8, 1, 1, driver.FilterNearest, driver.FilterNearest, driver.BufferBindingShaderStorageRead)
+	if err != nil {
+		g.Release()
+		return nil, err
+	}
+	g.output.nullMaterials = null
 	shaders := []struct {
 		prog *computeProgram
 		src  shader.Sources
@@ -1275,9 +1283,11 @@ func (g *compute) render(images *textureAtlas, dst driver.Texture, cpuDst cpu.Im
 		if !g.useCPU {
 			g.ctx.BeginCompute()
 			g.ctx.BindImageTexture(kernel4OutputUnit, dst, driver.AccessWrite, driver.TextureFormatRGBA8)
+			img := g.output.nullMaterials
 			if images != nil {
-				g.ctx.BindImageTexture(kernel4AtlasUnit, images.image, driver.AccessRead, driver.TextureFormatRGBA8)
+				img = images.image
 			}
+			g.ctx.BindImageTexture(kernel4AtlasUnit, img, driver.AccessRead, driver.TextureFormatRGBA8)
 		} else {
 			*g.output.descriptors.Binding2() = cpuDst
 			if images != nil {
@@ -1478,6 +1488,7 @@ func (g *compute) Release() {
 		Release()
 	}
 	res := []resource{
+		g.output.nullMaterials,
 		&g.programs.elements,
 		&g.programs.tileAlloc,
 		&g.programs.pathCoarse,