gpu: introduce driver.Device.PrepareTexture and use it in renderers

Vulkan textures (VkImage) are always in a particular layout, where each
layout is optimized for a particular use (transfer, sampling, compute
storage). Vulkan allows layout transitions everywhere except inside
render passes. This change adds driver.Device.PrepareTexture for
instructing the driver to switch a texture to a layout for sampling
in preparation for using it in a render pass.

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

gpu/headless/driver_test.go

@@ -75,9 +75,9 @@ func TestInputShader(t *testing.T) {
 	defer pipe.Release()
 	buf, err := b.NewImmutableBuffer(driver.BufferBindingVertices,
 		byteslice.Slice([]float32{
-			0, .5, .5, 1,
-			-.5, -.5, .5, 1,
-			.5, -.5, .5, 1,
+			0, -.5, .5, 1,
+			-.5, +.5, .5, 1,
+			.5, +.5, .5, 1,
 		}),
 	)
 	if err != nil {