fix(dat): render DXTL textures opaque

DXTL's fourth channel is luminance, not transparency - it premultiplies
the colour and the texture is opaque. ProcessImageFile kept that channel
as alpha, so DXTL content (all skill icons) rendered at ~35-60% opacity:
washed out and semi-transparent. Force alpha to 255 after premultiplying.
Confirmed against real skill icons (0x154c4, 0x6155, both ATEXDXTL) which
now decode to full opacity; visually verified via exported DDS.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 3vcloud

GWToolboxdll/Utils/GwDat/AtexReader.cpp

@@ -341,13 +341,16 @@ DatTexture ProcessImageFile(unsigned char* img, int size)
         tex_type = TextureType::BC5;
         break;
     case 'L':
+        // DXTL's fourth channel is luminance, not transparency: premultiply the
+        // colour by it and leave the texture opaque (skill icons etc. are DXTL).
         AtexDecompress((unsigned int*)img, size, 0x12, r, (unsigned int*)output.data());
         image = ProcessDXT5((unsigned char*)output.data(), r.xres, r.yres);
         for (int x = 0; x < r.xres * r.yres; x++)
         {
             image[x].r = (image[x].r * image[x].a) / 255;
             image[x].g = (image[x].g * image[x].a) / 255;
             image[x].b = (image[x].b * image[x].a) / 255;
+            image[x].a = 255;
         }
         tex_type = TextureType::BC5;
         break;