arbitrary rotation of images

Bug 1750527 reports that, with usetex, it is not possible to rotate text or
ticklabels by 45 degrees. It looks like this limitation is caused by
_backend_agg's draw_image method, which does not accept an angle argument,
while draw_text_image does. I am out of my depth in _backend_agg.cpp, can
draw_image be updated to accept an angle?