This patch modifies the GLSL linker to assign additional slots for
varying variables used by transform feedback, and record the varying
slots used by transform feedback for use by the driver back-end.

This required modifying assign_varying_locations() so that it assigns
a varying location if either (a) the varying is used by the next stage
of the GL pipeline, or (b) the varying is required by transform
feedback.  In order to avoid duplicating the code to assign a single
varying location, I moved it into its own function,
assign_varying_location().

In addition, to support transform feedback in the case where there is
no fragment shader, it is now possible to call
assign_varying_locations() with a consumer of NULL.

Reviewed-by: Marek Olšák <maraeo@gmail.com>
Tested-by: Marek Olšák <maraeo@gmail.com>