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/intersect.vert

@@ -10,7 +10,7 @@ layout(location = 1) in vec2 uv;
 layout(binding = 0) uniform Block {
 	vec2 scale;
 	vec2 offset;
-} uvparams;
+};
 
 layout(location = 0) out vec2 vUV;
 
@@ -18,5 +18,5 @@ void main() {
   vec2 p = pos;
   p.y = -p.y;
   gl_Position = vec4(p, 0, 1);
-  vUV = uv*uvparams.scale + uvparams.offset;
+  vUV = uv*scale + offset;
 }