Add support for subpixel font rendering.

[mrDIMAS]

Subpixel font rendering results in better font quality, however it is much harder to implement correctly. The engine uses fontdue crate for rendering fonts and it already supports subpixel rendering. However produced glyph texture cannot be used as-is, instead it requires some juggling with colors in fragment shader to make it look good enough.

Straightforward implementation uses dual-source blending and it creates some ugly colored edges: