Don't save and restore graphic states around showGlyph in LegacyPDFStreamEngine

One of the major performance bottlenecks in text extraction was the

clone + push and the pop + clone operations on the graphic state before and after the call to showGlyph.

Not only it was slow to clone, it also consumes large amounts of memory making the garbage collector work harder.

When extracting text, showGlyph does not modify the graphic state so there's no need to save / restore the state.

The same could be true in general, not just for text extraction, but I do not understand the code well enough to decide.

I have only modified the behavior for the LegacyPDFStreamEngine, to be safe.

The showGlyph operation sounds like a read only operation, that should not modify anything.

I have the code ready and I will submit a patch and a review.