Fix built-in filter on buggy drivers.

OS X v10.7 GMA 950 driver reports gl_InstanceID on location != -1, even should built-ins should always be at -1.
Need to work around this in the gfx-rs...
Attributes started with gl_ are reserved:
"If the named attribute variable is not an active attribute in the specified program object or if name starts with the reserved prefix "gl_", a value of -1 is returned."
from https://www.opengl.org/sdk/docs/man3/xhtml/glGetAttribLocation.xml