#19 SDL_image backend rendering issue

[Paul Wise]

With the SDL_image backend, the sheild/damage bars on the left and right flash as they rotate.

I'm guessing this is due to a difference between the GLU mipmap building function and the duplicate of it in glpng.

This can be fixed by loading the texture using GL_CLAMP_TO_BORDER, but that causes another issue when super-sheilds are on and get drained.