After a roundabout discussion[1] with wlroots devs, it's become apparent
that subpixel text rendering (a.k.a. "ClearType") does not work properly
when rendering over a transparent background, as labwc currently does.
Basically it comes down to the fact that the color of semi-transparent
pixels (which is adjusted redder or bluer to compensate for RGB subpixel
alignment) depends somewhat on background color. When rendering over
transparency, the text engine doesn't know the intended background color
and can't adjust the pixel colors correctly.
With Pango/Cairo, the end result can range from grayscale rendering (no
subpixel rendering at all) to wrong/oversaturated colors (for example,
bright pink pixels when rendering white text on blue background).
This change solves the issue by first filling the text buffer with an
opaque background color before rendering the text over it. Currently,
this is easy since the background is always a solid color. It may be a
little more complex (but doable) if we implement gradients in future.
Note that GTK 4 (and to some degree, recent versions of Microsoft
Windows) avoid this issue by disabling subpixel rendering altogether. I
would much prefer that labwc NOT do this -- it results in noticeably
blurrier text on non-retina LCD screens, which are still common.
[1] https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3822
