plug-ins: fix comment explaining cairo_save().
Current comment was implying that PDF had some kind of fixed PPI, or that a 72 PPI was hardcoded in cairo. This is not at all what this is about. Cairo simply has a concept of user-space coordinates, and from what I gather, it seems that this is set as "point", the typographical unit of measure (not as a screen point). In such context, cairo_scale() is used to update the transformation matrix (which will transform from user to device units, i.e. pixels) so that we can draw with pixels. In other words, both the user and device units will be pixels in subsequent calls. It turns out that 1 inch == 72 points. This is not a PPI/DPI at all and the comment was completely misleading.
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