Fix issues raised by the GLES 1.1 conformance test

- Most of the fixes are about unhandled enums.
- Adding an unbindBuffer method to GLEScontext, and use it in the
 glDeleteBuffers call, so a buffer is not considered bound after
 it has been deleted.
- Handle the case where a call to glCompressedTexImage2D gets
 NULL as its data (in which case the uncompressTexture function
 should only calculate the output format, but not attempt to
 uncompress the NULL data).
- A few segfaults.

Change-Id: I6a856ea6da1be3b15b41140d6383508a6803897c
6 files changed