Fix menu icons offset by recent Window menu additions
Fix toolbars not repainting after minimize/restore
Small tweaks to GDI+ resize code

Setting an all-black background on the destination layer prior to resize
solves previous issues with premultiplied alpha being jumbled
post-resize.

Also, when using GDI+ to save images to file, the calling function
should handle the un-premultiplication of 32bpp data.  As such, the GDI+
function now explicitly assumes it is being given unpremultiplied data,
rather then attempting to convert it itself.