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/headless/headless.go

@@ -47,7 +47,6 @@ func NewWindow(width, height int) (*Window, error) {
 		if err != nil {
 			return err
 		}
-		dev.Viewport(0, 0, width, height)
 		fboTex, err := dev.NewTexture(
 			driver.TextureFormatSRGBA,
 			width, height,