scaling test: Skip floating-point pixel formats with Cairo renderer
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).
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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
Edited by Simon McVittie