MF2_LINKDRAW support for OpenGL

Adds MF2_LINKDRAW support for sprite drawing in OpenGL. That fixes Tails's tails appearing in front of the shields and makes GFZ3's laser more accurate to the software renderer.

The drawing order is adjusted and the z-buffer writes of the linkdraw sprites are postponed to the end of sprite drawing so sprites drawn afterwards will be on top even if the real sprite location differs.

Drop shadow rendering also had to be adjusted for correct appearance.

The implementation can be simplified once the renderer is changed to draw all transparent surfaces and sprites together back to front instead of the current code where they are drawn separately.