draw: copy over prim id header in flatshade stage when emitting lines
draw: copy over prim id header in flatshade stage when emitting lines

Just like we do for tris (det shouldn't matter at this point, however
can have flags for things like line stipple reset).

No piglit change, it would fail line stippling tests if the flatshade
stage were run, which will happen with the next commit.

Reviewed-by: Jose Fonseca <61b5a29db2c0650f7c8510f93eece059c1b09320@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler