LibGfx: Improve glyph rendering speed for vector fonts

The glyph bitmap is a grayscale image that is multiplied with the
requested color provided to `Gfx::Painter::draw_glyph()` to get the
final glyph bitmap that can be blitted.

Using `Gfx::Color::multiply()` is unnecessary however: by simply taking
the destination color and copying over the glyph bitmap's alpha value,
we can prevent four multiplications and divisions per pixel.

In an artifical benchmark I wrote, this improved glyph rendering
performance by ~9%.

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