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/metal/metal_darwin.go

@@ -931,6 +931,8 @@ func (p *Pipeline) Release() {
 	*p = Pipeline{}
 }
 
+func (b *Backend) PrepareTexture(tex driver.Texture) {}
+
 func (b *Backend) BindTexture(unit int, tex driver.Texture) {
 	t := tex.(*Texture)
 	if enc := b.renderEnc; enc != 0 {