WME: Sprite background is not fully transparent if AlphaColor is set

[lolbot-iichan]

Discovered this issue at FoxTail game, that is using dinamic lighting on PNG sprites with transparent background.

I have exctacted sprite lighting source code and made a simple test game to demonstrate this issue without running FoxTail which is not fully supported by ScummVM yet: ​https://github.com/tobiatesan/wme_testsuite/tree/master/light_test/packages

Screenshots are attached. Resulting colot difference is not big (like #FFDED5 vs #FFDAD5), however you could see and feel this rectangle moving around character sprite both in test game and FoxTail.

Tested on latest ScummVM from dev branch and ScummVM 2.1.0git3428-gc518691d7c (Aug 12 2018 19:04:49).

[lolbot-iichan]

See ​https://github.com/scummvm/scummvm/pull/1664 pull request

[Filippos Karapetis <bluegr@…>]

In c4dc251f:

GRAPHICS: Fix fully transparent pixel blit

In BLEND_NORMAL mode with color != 0xffffffff, blending fully
transparent pixel was resulted in slightly modifying some background
colors, because old value X was a bit different from new value (X*255>>8).

This fixes defect #10686 WME: Sprite background is not fully transparent
if AlphaColor is set