When drawing processed vertices with the fixed function pipeline the
projection matrix is set up to map y values from 0 to height to 1.0;
-1.0(gl and d3d coord systems are flipped). This moves the y axis to
the bottom of the drawing area. When later on the y inversion matrix
is applied for offscreen rendering, the coordinate system will get
flipped out of the viewport.

This patch sets the Y range up upside down when using offscreen
rendering, so the invymat will flip it to the correct position. This
has to happen before the 0.375 pixel correction.