interlude for color management

This MR depends on !582 (merged) which is the reason this is a draft initially.

This is an arbitrary collection of small clean-up like changes:

• gl-renderer: do not unbind the context on output destroy
• Revert "gl-renderer: Make dummy surface current after all outputs are gone"
• gl-renderer: require GL ES 3.0 for color management
• gl-renderer: define fragment shader compile_const
• gl-renderer: use shadow framebuffer automatically
• Revert "compositor: add weston.ini option use-renderer-shadow"
• libweston: remove weston_output_set_renderer_shadow_buffer()

Perhaps the most important bit is creating the shadow framebuffer automatically if needed in gl-renderer, which means the weston.ini option use-renderer-shadow can be removed.