meta: Add a meta implementation of GL_ARB_clear_texture

Adds an implementation of the ClearTexSubImage driver entry point that tries
to set up an FBO to render to the texture and then calls glClear with a
scissor to perform the actual clear. If an FBO can't be created for the
texture then it will fall back to using _mesa_store_ClearTexSubImage.

When used in combination with _mesa_store_ClearTexSubImage this
should provide an implementation that works for all DRI-based drivers. However
as this has only been tested with the i965 driver it is currently only enabled
there.

v2: Only enable the extension for the i965 driver instead of all DRI drivers.
    Remove an unnecessary goto. Don't require GL_ARB_framebuffer_object. Add
    some more comments.