Switch default glsl shader to nearest.

As polled on bug 415, most people prefer the nearest shader. Since
there are still some quality issues with the hqscale shader, and
the UI switcher isn't implemented yet, make nearest the default.
Replace hqscale with hq4x implementation.

Reduce blurring and work better in 1:1 cases. Improves quality.
Merge remote-tracking branch 'alicelr/master'