Filling in missing alpha for blending operations

Fixes  #2012, fixes #2275. It's the same problem as #2030 - Java won't
set the high bits when RGB, returns 0 for alpha in
BlendingContext#compose(Raster, Raster, WritableRaster)
Merge pull request #2645 from JakubValtar/blend-fix

Fixes blending operations in Java2D