DRW: Remove screen_vecs

These were only a normalized copy of the XY axes of the inverse viewmat.
But since the viewmatrix is always normalized we can use it directly.

source/blender/draw/intern/draw_manager.h

@@ -598,7 +598,6 @@ typedef struct DRWManager {
   struct GPUFrameBuffer *default_framebuffer;
   float size[2];
   float inv_size[2];
-  float screenvecs[2][3];
   float pixsize;
 
   struct {