WIP: shader: Emit winsys_adjust_y only once per FS shader

If more then one input variable is tagged as requiring winsys_adjust_y then this variable would be declared more then once and consequently compiling the shader would fail.

Keep track of whether the variable was already declared to make sure it is emitted only once.

Closes: #72 (closed)

Signed-off-by: Gert Wollny gert.wollny@collabora.com