Support sub-pixel anti-aliasing and fix blurry text issues

[Tyriar]

This PR does the following:

• Characters are now drawn to an integer grid, but cell width is now floored. Originally it was ceiled which made the text look odd, and then a floating point number was used which makes the CharAtlas optimizations look blurry. Now characters look great whether they're cached or uncached.
• Background and foreground layers are now merged, this was necessary to support sub-pixel anti-aliasing as the effect only applies when drawn only an opaque background.
• Bold is now supported for uncached text.

Fixes #988
Fixes #987
Fixes #973

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[Tyriar]

I just covered the following cases:

• Draw background on text layer before the canvas is shown to prevent a black flash when the background isn't #000
• Draw background on char atlas to properly support non-#000 backgrounds
• Redraw background when the canvas is resized

[coveralls]

Coverage decreased (-0.2%) to 70.003% when pulling b225866 on Tyriar:blurriness_fix into 136c82f on sourcelair:v3.

[coveralls]

Coverage decreased (-0.03%) to 70.127% when pulling c2b765f on Tyriar:blurriness_fix into 136c82f on sourcelair:v3.

[coveralls]

Coverage decreased (-0.03%) to 70.127% when pulling c2b765f on Tyriar:blurriness_fix into 136c82f on sourcelair:v3.

[coveralls]

Coverage decreased (-0.1%) to 70.035% when pulling aca7ab6 on Tyriar:blurriness_fix into 136c82f on sourcelair:v3.

[coveralls]

Coverage decreased (-0.04%) to 70.116% when pulling fee381e on Tyriar:blurriness_fix into 136c82f on sourcelair:v3.

[coveralls]

Coverage decreased (-0.03%) to 70.127% when pulling 8bdc8fb on Tyriar:blurriness_fix into 136c82f on sourcelair:v3.

[mofux]

This looks really good now. There are two remarks I want to make:

1. After this is merged, we should open / re-open an issue to merge the selection layer logic into the TextRenderLayer, or come up with another approach to have the selection drawn between foreground and background.

2. Using Chrome's Rendering Inspector and enabling Paint Flashes, I can see that almost the whole line is now repainted if I type a regular letter like "w". Only exception seems to be a space character. Is this intentional?

Update: Forget about 2). This also happens in the current v3 branch. It only happens in zsh but not in bash though - probably zsh sends not only the letter but the whole line if the prompt changes.

[Tyriar]

The paint flashing in 2 isn't actually re-drawing the whole line, Chrome's just saying "things in this rectangle are changing"