This adds a heuristic for freeing shader programs that have not been
needed for a while. The intention is to stop Weston accumulating shader
programs indefinitely, especially in the future when color management
will explode the number of possible different shader programs.

Shader programs that have not been used in the past minute are freed,
except always keep the ten most recently used shader programs anyway.
The former rule is to ensure we keep shader programs that are actively
used regardless of how many.  The latter rule is to prevent freeing too
many shader programs after Weston has been idle for a long time and then
repaints just a small area. Many of the shader programs could still be
relevant even though not needed in the first repaint after idle.

The numbers ten and one minute in the above are arbitrary and not based
on anything.

These heuristics are simpler to implement than e.g. views taking
references on shader programs. Expiry by time allows shader programs to
survive a while even after their last user is gone, with the hope of
being re-used soon. Tracking actual use instead of references also
adapts to what is actually visible rather than what merely exists.

Keeping the shader list in most recently used order might also make
gl_renderer_get_program() more efficient on average.

last_repaint_start time is used for shader timestamp to avoid calling
clock_gettime() more often. Adding that variable is an ABI break, but
libweston major has already been bumped to 10 since last release.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>