Only respect the text color's alpha component for Bitmap glyphs.

[mstange]

Fixes #1780.
Fixes #1876.

This restores a chunk of code that was added in #1754 and removed in #1816, with the difference that I use color.a as the alpha component instead of 1.0.

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This change is 

[mstange]

r? @glennw

[Gankra]

In the context of AppleColorEmoji, this change seem reasonable and correct.

However a big part of #1816 was refactoring how bitmap fonts actually work. Was that all just for that one font? Are there other fonts that this mode now applies to and wouldn't be correct?

[Gankra]

As a minor style point, this could just be a ternary:

let color = if font.render_mode == FontRenderMode::Bitmap { ... } else { ... };

I don't expect this should need to be updated for other font modes.

[mstange]

I don't know the answers to these questions. I'm hoping Glenn does :)

[mstange]

I can add a new FontRenderMode value called Color and use that for Apple emojis. How does that sound, @glennw?

[glennw]

@mstange How about instead we have something like (with better naming perhaps):

enum BitmapColorMode {
    Color,
    Alpha,
    // None if we need in the future to ignore color *and* alpha??
}

enum FontRenderMode {
    ...
    Bitmap(BitmapColorMode)
}

This allows us to specify how to treat the color for a bitmap font, without needing a new font render mode.

[mstange]

@lsalzman had some concerns about this change. He is going to come up with a better fix.