Silence a warning about a change of sign.

Silence a warning about a change of sign.

GLuint is, like the name says, unsigned. To store -1 in it, a change
of sign happens. Silence the warning by casting the -1 to GLuint
first.

Change-Id: I165a32c486358a60f7c5fd0c439204ed2f9f7f16
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>