Fix WMO textures importing into Blender as black

[Wobblucy]

WMO surfaces import into Blender rendering solid black even though the exported textures are fine. On the dungeon this was found on, the texture covering a third of the model has a good colour channel and an all-zero alpha channel.

Cause

Opaque game textures often carry an unused alpha channel that sits at zero. Blender defaults loaded images to STRAIGHT alpha, which multiplies the colour by alpha when sampling — so a texture with good colour and zero alpha reaches the shader as black.

Rendering the affected texture as flat emission makes it plain:

alpha_mode	sampled result
STRAIGHT (default)	black, median 0/255
CHANNEL_PACKED	the actual art, median 88/255

Several material builders in import_wowobj.py already set CHANNEL_PACKED on the images they load, but the paths that go through loadImage without it — including the WMO shader 20 builder — kept the default and rendered black.

Change

loadImage sets alpha_mode = 'CHANNEL_PACKED' on every image it loads. This uses the colour as-is and — unlike NONE — keeps the alpha channel readable for the paths that use it as a blend mask or height source. One shared load point, all builders covered.

Verification

• Flat-emission A/B render of the zero-alpha texture: STRAIGHT → black, CHANNEL_PACKED → correct colour.
• Fresh import of the affected dungeon WMO and a terrain tile through the patched addon: previously-black surfaces render with their actual art.