Mesa has a bug that causes a crash due to a NULL pointer dereference
with the R200 driver when making a context current that has
GL_FRAGMENT_SHADER_ATI enabled. This patch works around this bug by
making sure that GL_FRAGMENT_SHADER_ATI is disabled before deactivating
a context, and reactivates it afterwards. The context manager keeps
GL_ATI_FRAGMENT_SHADER generally enabled, except if the context is in 2D
blit mode.