fix small bugs

some conversions from opaque colors to colors with alpha did
set the alpha value to 0. It should have been 0xFF.

Color8A wasn't properly exported before.
fix unit tests for blitbuffer API

tests for color equality now use "==" as operator instead of an
isEqual() function.

Also, a test for blending colors using alpha values is added.
Merge pull request #233 from hwhw/master

fix bugs in blitbuffer implementation, fix unit test spec