Replace last glDrawBuffer call with glDrawBuffers(1, ...)

Replace last glDrawBuffer call with glDrawBuffers(1, ...)

glDrawBuffer is only available in Desktop OpenGL, while the equivalent
glDrawBuffers is valid also for GLES.

Just defining glDrawBuffer as an empty macro is obviously not the right
solution, as the call is also required on GLES. This also causes
a compilation failure - GL.h may be included via GLX.h on X11 platforms,
and the glDrawBuffer prototype declaration becomes malformed.

Gbp-Pq: Name 81_allow_gles_platforms.patch