Select charmap when loading fonts and add workaround for windows symbol fonts.
Use special handling for Windows/Symbol charmap only with charcodes >= 0xf000.

Some fonts with symbol encoding still have the charcodes in the ASCII
area instead of Unicode page 0xf000. To make those work too, the special
handling is only enabled in case the first charcode is >= 0xf000.
Add test cases for composition and symbol font handling.
Add example files for text() usage.

The test case images are currently not including the text as the example
test run does not include experimental features. This will need to be
updated once the text() feature is not experimental anymore.
Merge pull request #868 from openscad/text-fixes2

Text fixes (part 2)