Adds command line options:
--invert-y
--iy
(synonyms) which invert position.Y on vertex shader output. Handles these cases:
* Direct single variable return
* Member of direct returned struct
* Single variable output parameter
* Member of struct output parameter
API:
// Enables position.Y output negation in vertex shader
void TShader::setInvertY(bool invert);
Fixes#1173
