Based on !7680 (merged), please review that first (this MR needs some utility code from that one, and also some of its changes would conflict).
scaling test: Skip floating-point pixel formats with Cairo renderer
If the four chosen pixel values happen to have alpha values that average to 16/17, GTK will quantize that into 8bpp integer space as 16/17*255 = 240, but Cairo/Pixman does its quantization differently and gets 16/17*256 = 241.
Resolves: #6978