gpu: introduce render passes

Modern GPU API such as Metal and Vulkan use explicit render passes
and command buffers for recording rendering commands. They don't have
global state; each render pass starts with a clean set of bound
textures, pipeline etc.

Change our GPU abstraction to better match newer API and modify our two
renderers to explicitly describe their render passes.

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

gpu/internal/d3d11/d3d11_windows.go

@@ -789,7 +789,13 @@ func (f *Framebuffer) ReadPixels(src image.Rectangle, pixels []byte, stride int)
 	return nil
 }
 
-func (b *Backend) BindFramebuffer(fbo driver.Framebuffer, d driver.LoadDesc) {
+func (b *Backend) BeginCompute() {
+}
+
+func (b *Backend) EndCompute() {
+}
+
+func (b *Backend) BeginRenderPass(fbo driver.Framebuffer, d driver.LoadDesc) {
 	b.fbo = fbo.(*Framebuffer)
 	b.ctx.OMSetRenderTargets(b.fbo.renderTarget, nil)
 	if d.Action == driver.LoadActionClear {
@@ -799,6 +805,9 @@ func (b *Backend) BindFramebuffer(fbo driver.Framebuffer, d driver.LoadDesc) {
 	}
 }
 
+func (b *Backend) EndRenderPass() {
+}
+
 func (f *Framebuffer) Release() {
 	if f.foreign {
 		panic("framebuffer not created by NewFramebuffer")