fix: use mask alpha channel for blending in ItemViewport shaders

Simplify the mask blending logic in quickitemviewport shaders by using only
the mask's alpha channel instead of multiplying with the entire mask color.
This ensures correct color output when the mask texture contains non-neutral
RGB values and maintains consistency with the expected premultiplied alpha
behavior.

Log: Fix mask blending to use alpha channel only in ItemViewport shaders

fix: 修复 ItemViewport shader 中 mask 混合逻辑

简化 quickitemviewport shader 中的 mask 混合逻辑，仅使用 mask 的 alpha 通道
而非整个 mask 颜色值进行混合。这确保了当 mask 纹理包含非中性 RGB 值时输出
颜色正确，并与预期的 premultiplied alpha 行为保持一致。

Log: 修复 ItemViewport shader 中 mask 混合应仅使用 alpha 通道

PMS: BUG-306847

Author: Ivy233

qt6/src/shaders_ng/quickitemviewport-opaque.frag

@@ -44,9 +44,6 @@ void main()
         discard;
 
     lowp vec4 tex = texture(qt_Texture, qt_TexCoord);
-    // The input texture is already premultiplied. Multiplying by tex.a again
-    // darkens the partially covered edge pixels and makes corner artifacts more
-    // visible on transparent windows under fractional scaling.
-    tex *= mask_tex;
-    fragColor = tex;
+
+    fragColor = tex * mask_tex.a;
 }

qt6/src/shaders_ng/quickitemviewport.frag

@@ -46,9 +46,5 @@ void main()
 
     lowp vec4 tex = texture(qt_Texture, qt_TexCoord);
 
-    // The input texture is already premultiplied. Multiplying by tex.a again
-    // darkens the partially covered edge pixels and makes corner artifacts more
-    // visible on transparent windows under fractional scaling.
-    tex *= mask_top_left * mask_bottom_left * mask_top_right * mask_bottom_right;
-    fragColor = tex * ubuf.opacity;
+    fragColor = tex * mask_tex.a * ubuf.opacity;
 }