gpu: switch from uniform buffers to combined push buffers

Vulkan doesn't support changing uniforms during a render pass. However,
push constants *can* be changed. The gio-shader repository was changed
to use push constants instead of uniforms, this change implements the
corresponding driver and renderer change.

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

go.mod

@@ -10,5 +10,5 @@ require (
 
 require (
 	gioui.org/cpu v0.0.0-20210817075930-8d6a761490d2
-	gioui.org/shader v1.0.2
+	gioui.org/shader v1.0.3
 )