Refactored code to get glyph from a codepoint

[bassklampfe]

This fixes #490
It also is a preparation for UTF8 support in my fork, but this fix itself is general

• Describe the scope of your change--i.e. what the change does and what parts
  of the code were modified.
  Refactored common code to get GFXglyph from GFXfont. It fixes the issue accessing uint16_t values in GFXfont using (wrong) pgm_read_byte
  By the way it also fixes drawChar, which did not check for limits at all

• Describe any known limitations with your change.
  on atmega platform this patch increases code size by 54 bytes.

• Please run any tests or examples that can exercise your modified code.
  Tested with example from 'Adafruit ST7735 and ST7789 Library' and also with my own code (to be published somewhere in the future)

[bassklampfe]

I did it wrong (old subversion user still learning git). trying to abort this pull request and do it right