[FlareCoding]

Summary

Every blend in libstlxgfx, userland's 2D drawing library, makes its pixel opaque, so a picture drawn onto a
transparent surface loses its coverage (the share of each pixel it covers), which it needs to be blended onto the
screen later, and a surface flag keeps that coverage.

Changes

• Keep a picture's coverage: on a surface whose flags field holds STLXGFX_SURFACE_STRAIGHT_ALPHA, the
  blending calls (stlxgfx_blend_coverage, stlxgfx_fill_rect_blend, stlxgfx_blit_alpha,
  stlxgfx_blit_rounded_alpha, stlxgfx_blit_arc_corner and stlxgfx_draw_text) store straight alpha (a color not
  multiplied by its alpha), so a draw onto a transparent pixel stores its color and alpha. A draw onto a partly
  covered pixel stores the coverage of both draws together, rounded up, with the mean of both colors weighted by the
  coverage each shows, while opaque pixels, like every surface without the flag, blend exactly as before.
• Test library code in CI: make host-test builds libstlxgfx's tests in userland/lib/libstlxgfx/tests/ with the
  build machine's clang and runs them there, outside Stellux, and a "Host tests" workflow runs the target on every
  pull request.
• Avoid undefined behavior in arc corners: stlxgfx_blit_arc_corner casts its 8-bit coverage to uint32_t
  before shifting it 24 bits left, since C promotes it to a signed int, which the shift overflows for coverage
  above 127.
• Tests: straight_alpha.test.c checks each blending call on flagged surfaces: a draw onto alpha 0 stores its color
  and alpha, that picture blended once onto an opaque surface matches the same draw made there, a draw onto alpha 1 to
  254 follows the rule that stlxgfx_blend_straight states, with each channel between both colors, and one onto alpha
  255 matches an unflagged surface. In QEMU on x86_64 and aarch64, with a terminal, the demo app windemo and the
  settings app open, the desktop below the top bar is byte-identical to the base commit's, b36caf7 (the landing
  record's reviews/gpu-compositor/14a2-desktop-x86_64.png and -aarch64.png show both desktops, the base on the left,
  and 14a2-buttons-x86_64.png and -aarch64.png the settings app's buttons zoomed 4 times, the base above).