scale via MonoGame instead of static images

BQ uses three different sizes of images (x1, x2, and x3) to handle
scaling. Letting MonoGame handle up-scaling results in pixel-identical
render, using SamplerState.PointClamp. This means less static images in
the code and far simpler scaling logic.
Merge pull request #7 from half-ogre/half-ogre/better-scaling

Scale via MonoGame instead of using BQ's static images