Import constness fix from gl3.h into gl2.h.

Import constness fix from gl3.h into gl2.h.

This change has been made in Khronos SVN for GLES2 already, however it
is mildly controversial and discussion is on-going.

This change is necessary for Android because otherwise applications
cannot include both GLES2/gl2.h and GLES3/gl3.h as the glShaderSource()
prototype mismatches. Native programs may include framework headers
(which now include GLES3 headers only) but already have included GLES2
headers, creating an incompatibility.

Change-Id: I70a4075e079ee063e76cef5a227abb0ae70aebb4