drm_hwcomposer: lazily generate shaders for GLWorkerCompositor

drm_hwcomposer: lazily generate shaders for GLWorkerCompositor

This still generates the first shader at GLWorkerCompositor::Init() to check
for shader generation errors. It's the only time that shader compilation errors
will be shown on the log.

This change shortens GL initialization time from 2.8 seconds to about 450
milliseconds. Besides improving boot time by 2.3 seconds, less fence timeouts
should happen at boot time due to HWC blocking.

TEST=run and see no display glitches or glworker related errors in logs
BUG=chrome-os-partner:46739

Change-Id: Ia264e3f73a0ebe8558165ae3c215777ef6339ecc