core: tests: Test for invalid context pixel type.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
pywrap: backends: Change UpdateRect API

The UpdateRect() now takes four numbers rather than array.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
doc: Slight update in the python backend docs.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
demos: py_simple: Add demo for alpha channel blit.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
core: Blit: Implement blit with alpha channel.

This is first version, more to come.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
core: ContextConvert: Fill the allocated buffer.

Since alpha channel blits started to work,
the image will be alpha blended with random
mess in the allocated buffer. Fix it by
filling the buffer with zeroes first which
restores the previous behavior.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
core: Pixel: Add GP_PixelHasAlpha().

Signed-off-by: Cyril Hrubis <metan@ucw.cz>