glsl_to_tgsi: invert gl_SamplePosition.y for the default framebuffer

Fixes dEQP-GLES31.functional.shaders.sample_variables.sample_pos.correctness.default_framebuffer
with --deqp-gl-config-name=rgba8888d24s8ms4

Cc: 18.1 18.2 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit f0cd7dbc)