The old code transformed the top-left and bottom-right corner of the rectangle
to device space and calculated a rectangle based on these two points. However,
if you rotate a rectangle by 45°, these two points will be directly above each
other and thus the old code would calculate a width of 0.
Fix this by transforming all four corners of the rectangle into device space and
calculating a rectangle based on this.
Signed-off-by: Uli Schlachter <psychon@znc.in>
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