Support for storage buffers

[kvark]

We are exploring the use of storage buffers in WebRender, and glsl optimizing stage unfortunately doesn't recognize them. Example syntax we use:

layout(std140, binding = 0) buffer bInstances {
    ivec4 bInstanceData[];
};

This is code in GLES 3.1, and under "GL_ARB_shader_storage_buffer_object" extension in GL.