Instead of having separate main() and gl_main() functions in HLSL
shaders, add initializing outputs and inputs directly to the main
function that's in the AST.

This works around some HLSL bugs and should not introduce name
conflicts inside main() since all the user-defined variables are
prefixed.

BUG=angleproject:2325
TEST=angle_end2end_tests

Change-Id: I5b000c96aac8f321cefe50b6a893008498eac0d5
Reviewed-on: https://chromium-review.googlesource.com/1146647Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>