Map glyph bitmap in LookUpGlyphBitmap even when unsuccessful
Map glyph bitmap in LookUpGlyphBitmap even when unsuccessful

CFX_FaceCache::LookUpGlyphBitMap should remember its unsuccessful glyph
bitmap calculations. This is especially important if there are a lot of
repeated requests for the same glyph with large bitmaps (which means
long calculations in freetype). With this CL, the pdf in the bug below
will be rendered quickly.

BUG=chromium:638856

Review-Url: https://codereview.chromium.org/2338883004

DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium