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
use-renderer-shadow can be removed.