mesa: Move RasterDiscard to toplevel of gl_context.

mesa: Move RasterDiscard to toplevel of gl_context.

Previously we were storing the RasterDiscard flag (for
GL_RASTERIZER_DISCARD) in gl_context::TransformFeedback.  This was
confusing, because we use the _NEW_TRANSFORM flag (not
_NEW_TRANSFORM_FEEDBACK) to track state updates to it, and because
rasterizer discard has effects even when transform feedback is not in
use.

This patch makes RasterDiscard a toplevel element in gl_context rather
than a subfield of gl_context::TransformFeedback.

Note: We can't put RasterDiscard inside gl_context::Transform, since
all items inside gl_context::Transform need to be pieces of state that
are saved and restored using PushAttrib and PopAttrib.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>