Plugins (alphablend): Fix blending and associated crashes due to buffer overruns

[kaixiong]

The alphablend morph uses the blending routines visual_alpha_blend_nn() wrongly. It passes the pixel buffer size in bytes instead of pixel count for the size parameter. Since buffer size is always equal or greater than pixel count (bytes per pixel >= 1 and row padding >= 0), this causes buffer overruns.

[hartwork]

@kaixiong two small things:

[hartwork]

get_blend_func can return NULL. If we keep allowing it to, we likely need a check for NULL somewhere around here?

[kaixiong]

The default case is unreachable and largely included to silence compiler warnings about enum values unaccounted for (e.g. VISUAL_VIDEO_DEPTH_NONE and VISUAL_DEPTH_NONE_ALL).

We don't have unreachable() in C99 or C11 so I returned a NULL instead. Would you prefer abort()?

[hartwork]

@kaixiong I would favor some flavor of abort or assert — something with a message —, yes, maybe using libvisual's log_and_exit? (Maybe we need one more check for that for lv_checks.h.)

[kaixiong]

@hartwork I've added a critical log message followed by abort().

I spent some time thinking about the general approach of a portable unreachable() function that defaults to log-and-abort in debug builds... but I've ultimately concluded that for now, we should keep the change small for this PR 😄