LibGL+LibSoftGPU: Add GL_ADD Texture Environment

Author: https://github.com/Quaker762
Commit: c9f44c746a
Pull-request: https://github.com/SerenityOS/serenity/pull/13271
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/EWouters
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/gmta
Reviewed-by: https://github.com/sunverwerth ✅

Userland/Libraries/LibGPU/SamplerConfig.h

@@ -34,6 +34,7 @@ enum class TextureEnvMode {
     Modulate,
     Replace,
     Decal,
+    Add,
 };
 
 struct SamplerConfig final {