Type the remaining untyped class constants (a PHP 8.3 feature) in
DominantPaletteAnalyzer and the VibrantMuted theme definition, and add
parameter and return types to closures and arrow functions in src and
tests that were still missing them.

Stream resources keep their docblock-only types since PHP has no native
resource type.

The background was painted on the new canvas and a transparent cutout
polygon drawn where the rotated image goes, while imagerotate() filled
the new areas with a transparent color. The polygon does not match the
area covered by imagerotate() exactly, so a seam of transparent pixels
was left along the edges of the rotated image on opaque backgrounds.

Let imagerotate() fill the new areas with the background color and copy
the result without blending, which makes the cutout unnecessary.

libgd fills pixels of the transparent color with this color when
rotating, so images with a transparent color (GIF, indexed PNG) are
first copied to a truecolor image without one. The background is no
longer allocated in the frame palette, which returned false on a full
palette and made imagerotate() throw a TypeError.