Removed shade drawing optimization

Why?
- This optimization has caused many glitches, it easy to break and
  we've done so a few times (technical debt.)
- It's unclear if not calling glClear() is still an optimization.
  Most graphics cards actually expect a glClear and are optimized
  to discard old frame buffers when the method is invoked

Change-Id: If1bb9b8bf4c907ca907dc1205ad05dabfefeba1b
Fixes: 113286511
Test: manual
Test: shalac@ is keeping an eye on the performance dashboard