If only glUniform is called between two renderings, the const buffers weren't
getting updated. Need to set the _NEW_PROGRAM flag in st_upload_constants()
as that's the dirty flag set by glUniform.
Fixes glean tapi2 test.
Provide an actual definition of the pipe_buffer struct, containing
the parameters used to create the buffer, and its refcount.
Shift refcounting buffers out of the winsys interface, similar to
surfaces & textures.
Rework pipebuffer/ to reflect the fact these changes, and also Michel's
reworking of the buffer interface.
The properties of a buffer represented by struct pipe_buffer_handle are now
basically constant over its lifetime. The state tracker gets to deal with any
more complex buffer semantics it may need to provide.
