Add subpixel positioning for GPU renderer
This MR adds subpixel positioning to the GPU renderer. It even works on hidpi, same as Cairo (unlike certain other renderers).
The positioning is disabled for non-affine transforms, because I thought using the non-offset glyph probably works best there.
I also added GSK_GPU_SKIP=glyph-align
to turn it off, not sure if that's the best name. I'm open for better suggestions.