`flip-y | rotate-N` transforms are equivalent to `flip-x | rotate-N`.
However, it is not intuitive to express transforms that flip the y axis
via flip-x transforms. This change adds convenience aliases to improve
code readability and keep code intuitive, which is going to be needed in
order to drop TextureTransforms flags.
These can be useful in case a QRect needs to be mapped. One could
convert the QRect into a QRectF and then back to QRect, but given that
these conversion helpers are present in rendering hot paths, it would
be great to be more efficient and also potentially loosing precision
because of floating point arithmetic.
