gpu,gpu/gl: implement shader uniform buffers

Emulate them for the OpenGL ES backend because 2.0 doesn't support uniform
buffers. The future d3d backend only supports uniform (constant) buffers.

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

gpu/shaders/common.inc

@@ -0,0 +1,4 @@
+#version 310 es
+
+void blah() {
+}