Improve performance by optimizing glyph lookup

[brdvd]

This PR improves the total runtime by optimizing the glyph lookup in resources. Some runtime data with a few pieces of different size:

Number of pages	Runtime before in s	Runtime after in s	Improvement in %
3	0.306	0.260	15
10	1.732	1.525	12
97	4.339	3.963	9

Note that after adding the possibility to use several fonts simultaneously, glyph lookup now involves two maps: one for loaded fonts and one for the glyph itself. Changes in this PR:

• We optimize the number of map lookups, mostly by replacing map::contains followed by map::at with a single map::find.
• We cache the glyph that was looked up last. This is helpful, since often the same glyph is requested several times consecutively, e.g. from calls of GetGlyphWidth and GetGlyphHeight.
• There are a few other (minor) code improvements in Resources.