0 sized masks are now possible

[augusto2112]

Issue: #491

[illume]

Looks like a few builds are failing. You can see them by clicking the "Details" link next to Appveyor and TravisCI.

Here's one issue on a 32bit python: https://ci.appveyor.com/project/pygame/pygame/build/1.0.584/job/wi2o2m3smiuim2wq

======================================================================
ERROR: test_zero_size_from_surface (pygame.tests.mask_test.MaskModuleTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Python27\lib\site-packages\pygame\tests\mask_test.py", line 460, in test_zero_size_from_surface
    zero_w_mask = pygame.mask.from_surface(pygame.Surface((0, 100)))
ValueError: Cannot create mask with negative size

[illume]

@augusto2112 I think it's good. Thanks :)

However, we don't have tests for the different methods operating on zero sized masks. If you want to add tests for those I can leave it open? Otherwise I'll merge it in?

[augusto2112]

@illume I'll add the tests 😄

[augusto2112]

@illume I'll do it tomorrow though because today I won't have enough time. Or you prefer, you can accept it and tomorrow I submit a new PR with the tests.

[augusto2112]

@illume I added a bunch of tests. I'm not sure what should happen when trying to scale a (0, 100) mask by (0, 2) for example. Currently it stays (0, 100).