opengl: fixed playback of 10bit content on Macs with OpenGL 1.4 drivers, notably using the GMA 950 chipset (close #5973)

We do so by forcefully disabling the 16bit shaders, which the chipset pretends to support, but actually doesn't.
(cherry picked from commit adcc3b14a16c712af51bfc9b7940921a9e6b3468)