Add missing initialization before requesting png data for scaled read

Initialize all values read to sensible default in case reading them
fails in read_image_scaled. It has already been done for a normal
image read

Task-number: QTBUG-32674
Change-Id: I19dccad7ef342f1b1bba6b513c04d3661d128a54
Reviewed-by: aavit <eirik.aavitsland@digia.com>
PNG: properly handle 1bpp greyscale images with alpha

PNG allows 1bpp greyscale images (PNG_COLOR_TYPE_GRAY) to have
an alpha key; so even in this case we need to inquiry if
the image has a transparency, and if so modify the color table
of the monochrome image accordingly.

Task-number: QTBUG-33503

Change-Id: Iab07c8f95ac8865269c48816e222645cdcb6bbc1
Reviewed-by: Nicolas Arnaud-Cormos <nicolas@kdab.com>
Reviewed-by: aavit <eirik.aavitsland@digia.com>