Furher fixes for a GLES 3.1 host

This series adds some more extension XXX required to the shader when running on a GLES 3.1 host fixing shader compilation when gl_Layer, imageAtomic*, and the sample keywords are used and enables GL_OES_draw_buffers_indexed if the host provides it.

I didn't check in detail whether it can be avoided that GL_OES_shader_multisample_interpolation is always emitted when SHADER_REQ_GPU_SHADER5 is set. If you think this should be done more fine-grained I can add another specific flag for the sample keyword.