adapt test spec to new blitbuffer API

isEqual() is now implemented as __eq metamethod (so "==" works)
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.