svg: fix chroma mapping on Big-Endian
The documentation [1] for CAIRO_FORMAT_ARGB32 says: "alpha in the upper 8 bits, then red, then green, then blue. The 32-bit quantities are stored native-endian."
So this is ARGB on Big-Endian and BGRA on Little-Endian.
[1] https://www.cairographics.org/manual-1.0.2/cairo-Image-Surfaces.html