remove frame background color property
reimplement frame background with special Patches
handle monochrome patches (reconstruct alpha)
rename fg/bg, adding 'Color' postfix for clarity
try known colors in alpha computation (slow)
only add successful fgColors to known set
try known colors first (again)
introduce MostFrequentlyUsedColors instead of set
prevent division by zero in unblend_alpha()
set FLAG_RECT as early as possible
extract detectAlpha() from image()
detect alpha before joining close rects
support recent popplerqt4 extension
Merge branch 'master' into try_known_colors
fix spelling of "occurrence"
Merge branch 'master' into try_known_colors
get rid of unused bgColor variable
simplify subarray() using QRect.getCoords
collect unique patches on ChangedRect level

* move key() from ChangedRect to Patch class
* collect occurrences at ChangedRect level (Patch only knows count)
only allow merging of rects with same occurrences