Fix ATI part of T45708, crash when enabling opensubdiv.

In fact exit was getting called because we
had an error in shader compilation:

Uniform buffer objects are in fact required.

Since it looks like original intent was to
write the shader against older GLSL version,
I will be adding an extension here instead
of a version.

Thanks to Anshu Arya for letting me borrow his machine through
VPN to do the debugging :)

intern/opensubdiv/gpu_shader_opensubd_display.glsl

@@ -28,6 +28,7 @@
 #extension GL_EXT_geometry_shader4 : enable
 #extension GL_ARB_gpu_shader5 : enable
 #extension GL_ARB_explicit_attrib_location : require
+#extension GL_ARB_uniform_buffer_object : require
 
 struct VertexData {
 	vec4 position;