rc = pmatrix:TransformPixelRectangle(rect)
rect using the matrix, then calculates the bounding box
of the transformed rectangle. The rect parameter must be a table that has
the x, y, width, and height fields initialized. The return table will
also have these fields initialized.
This function is useful for example when you want to draw a rotated Pango layout to an image buffer, and want to know how large the image should be and how much you should shift the layout when rendering.
For better accuracy, you should use pmatrix:TransformRectangle() on original rectangle in Pango units and convert to pixels afterward using pango.ExtentsToPixels()'s first argument.