qt4: win7 taskbar buttons are 16x16 since a3db0685

Close #9227
demux: avi: Ignore empty idx1 indexes for testing offset (fix #9248)

More broken avi files :/
contrib: Patch libpng to make it build with automake 1.12 and 1.11 again
tools: Check for libtool 2.4

This is required when building libpng.