opencl: Load cl_gl extension functions only if the extension is supported.

Load the extensions in a portable way.

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Don't use color_kernels, they are broken on mesa

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Disable image support check.

Works ok without

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>