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/internal/driver/driver.go

@@ -37,6 +37,7 @@ type Device interface {
 
 	BeginRenderPass(f Framebuffer, desc LoadDesc)
 	EndRenderPass()
+	PrepareTexture(t Texture)
 	BindProgram(p Program)
 	BindPipeline(p Pipeline)
 	BindTexture(unit int, t Texture)