Move to be more like Ghostscript in the handling of shapes.

When pushing a transparency group, MuPDF used to always create a shape
plane if there was one before. Ghostscript works differently, and
only creates a shape plane if we are in a non-isolated group.

This means that when blending an isolated group back to a non
isolated group, GS uses the "source alpha" in place of the "shape
alpha" to modify the non-isolated groups shape plane.

We update MuPDF to do the same here - it requires some new (small)
routines to do this new type of shape update, but means we carry
smaller amounts of data around overall.

Also, for what little remains of my sanities sake, this helps by
making it easier to compare the debug output from the 2 different
renderers.

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