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/cover.vert

@@ -8,11 +8,11 @@ layout(binding = 0) uniform Block {
 	float z;
 	vec2 scale;
 	vec2 offset;
-	vec2 uvScale;
-	vec2 uvOffset;
 	vec2 uvCoverScale;
 	vec2 uvCoverOffset;
-} uniforms;
+	vec2 uvScale;
+	vec2 uvOffset;
+};
 
 layout(location = 0) in vec2 pos;
 
@@ -22,7 +22,7 @@ layout(location = 1) in vec2 uv;
 layout(location = 1) out vec2 vUV;
 
 void main() {
-    gl_Position = vec4(pos*uniforms.scale + uniforms.offset, uniforms.z, 1);
-	vUV = uv*uniforms.uvScale + uniforms.uvOffset;
-	vCoverUV = uv*uniforms.uvCoverScale+uniforms.uvCoverOffset;
+    gl_Position = vec4(pos*scale + offset, z, 1);
+	vUV = uv*uvScale + uvOffset;
+	vCoverUV = uv*uvCoverScale+uvCoverOffset;
 }