[WIP] Fix #109: Implement basic subpixel rendering

[bryphe]

This change adds subpixel rendering for low DPI displays to improve the quality of the rendered text (important for editors like Onivim 2). The basic strategy is to render the font glyph at 3x resolution, and then use a kernel to sample each color channel individually.

The chosen strategy only works with solid color backgrounds, which is an acceptable trade-off for now (if there is a non-solid color background, we'll fallback to a non-subpixel strategy).

The initial kernel is very simple - sample each channel subpixel against the high-resolution glyph. There's better choices of kernel potentially (that require more texture reads), we can experiment and see what works well.

This fixes #109

TODO:

• Conditionally enable subpixel rendering only when low DPI and background color is solid
• Investigate using FT_Render_Mode_LCD with FreeType, along with various filters
• Remove hardcoding of scaling
• Handle pixel alignment - make glyph quad always pixel-aligned, but adjust subpixel offset for text (test in example)
• Try out different colors in example
• Add option in example to enable / disable subpixel rendering
• Should we be using GL_NEAREST instead of GL_LINEAR for sampling? Nope
• Experiment with alternate kernels - the 5-tap filter here looks promising: https://www.grc.com/cttech.htm
• Investigate filter options with Freetype (https://www.freetype.org/freetype2/docs/reference/ft2-lcd_filtering.html)

[bryphe]

Will close this out to be superseded by the Skia effort