gles2rice**: Revert to upstream by defining some GL constants.

**POSSIBLE DOWNSTREAM BEHAVIOR CHANGE**

Second opinion needed.  Not sure this changes anything.
gles2rice**: Revert flush/swap/clear mechanics to upstream.

**POSSIBLE DOWNSTREAM BEHAVIOR CHANGE**

Second opinion needed. Some quick tests didn't reveal any issues, but a
more thorough test/analysis is needed.
gles2rice**: Revert initial window sizing to upstream version.

**POSSIBLE DOWNSTREAM BEHAVIOR CHANGE**

Second opinion needed. Some quick tests didn't reveal any issues.
glrice: Pull gles2-specific code from downstream.
gl/gles2rice: Update author attribution in plugin version.