Closes #75

• Add FXAAPass and DDAAPass effects.
• Add renderer.ppaa property that can be set to 'default', 'none', 'fxaa', or 'ddaa'.
• This adds/removes the corresponding effect to the renderer.effect_passes.
• Add PYGFX_PPAA to control what 'default' resolves to. The primary use-case is to turn it off for image tests.

Line without aa, on a regular (no-hidpi) screen, with pixel ratio 1:

With pixel ratio 2 (the trick we use by default):

With ddaa and pixel ratio 1:

More comparisons: https://almarklein.github.io/ppaa-experiments/viewer.html#image=lines.png&algs=ori,ssaax8,fxaa3,ddaa2

DDAA is bases on an idea I had in the early days of Visvis. I revived the idea and worked on it outside of "Pygfx hours". A small improvement in the perceived smoothness (without getting blurry) can be a big win, because it means we can turn the aa of objects off, which removes a whole range of artifacts, and maybe even reduce the ssaa (pixel_ratio).

Differences of DDAA w.r.t. FXAA:

• Better smoothing for diagonal-ish edges.
• Uses similar edge-search technique as FXAA, does not skip pixels, but (therefore) searches a bit less far with default settings.
• Better preservation of small structures such as thin lines and parts of glyphs.
• More performant than FXAA (with default settings).

For more info see https://github.com/almarklein/ppaa-experiments