Catch failure of gdImageCreateTrueColor & terminate gracefully.
Add an option to print progress information while generating the map.
Inform the user of libgd and system limitations when applicable.

This is necessary, because libgd may fail, or print some
unintelligible message when these limits are reached.

Known limitations:
- libgd refuses to create images larger than approx INT_MAX (2^31-1)
  pixels (a little over 46300x46300), even on 64-bit systems.
- on a 32-bit system, in practise, the largest size that can be
  generated is a little over 24100x24100 pixels.
Remove superfluous '--' in README.rst
Fix alpha handling - libgd treats 0xff as transparent and 0x00 as opaque.