CompositeDropShadow in Common/Render/DropShadow.cpp blurs a premultiplied
RGBA image's alpha (with an optional offset) and composites the image
over it. The atlas's AddDropShadow and TextDrawer::AddDropShadow are now
thin wrappers that pick the parameters.
The blur scatters only the non-transparent texels, which makes atlas
shadows 3-7x faster than the old gather-with-clamping version. Atlas
output is unchanged apart from rounding (alpha differs by at most 1).
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
