reduce the amount of ctx save() and restore() when text is to scaled.

When a font is rendered which is subPixelAAEnabled it is possible for
each glyph the context is scaled and restored. This is inefficient in
two ways:
a) it is possible that the calculated scale is 1.
b) if the next glyph is scaled the same way as the last glyph, scale
   does not need to change at all.

Issue #3853 contains a PDF (http://www.msy.com.au/Parts/PARTS.pdf) which
did 76344 times ctx.save(), ctx.scale(), ctx.restore(). After the patch
it only does it 5586 times.

Unfortunately I was unable to get a stable benchmark for the improvement,
but it is roughly 10%-20% faster in the showText method.

PDFs which were not using the font.remeasure will not get any difference,
a boolean check was replaced with !==.