Along with being more accurate to the original, this fixes switching out
of fullscreen causing textures to disappear. This was caused by DirectX
destroying textures that are marked as render-targets when a device-loss
occurs. SDL2 doesn't do anything to recover them, unlike regular
textures, so my solution is just to avoid render-target textures
entirely.
On the upside, this should make drawing text to surfaces much faster,
since we're not reading back bitmaps from the GPU.
