It turns out it's sometimes more efficient to use
SkCanvas::drawPaint() with SkShader::Blend() used to blend bitmaps
together, rather than manually creating temporary SkImage
for the blending. This way it saves memory and it also performs
faster e.g. for tdf#134237, where when zoomed it processes only
relevant parts of the images instead of blending a whole enlarged
image).
Sadly in raster mode it is sometimes still faster to cache
the image (e.g. with tdf#134160), so keep the caching there as well,
for when useful.
Change-Id: I887ae330907100c21a0d152783fcd7e8ef230355
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/97238
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>
