LibGL+LibGPU+LibSoftGPU: Report texture env add extension

The Quake 3 port makes use of this extension to determine a more
efficient multitexturing strategy. Since LibSoftGPU supports it, let's
report the extension in LibGL. :^)

Userland/Libraries/LibGPU/DeviceInfo.h

@@ -18,6 +18,7 @@ struct DeviceInfo final {
     unsigned max_clip_planes;
     u8 stencil_bits;
     bool supports_npot_textures;
+    bool supports_texture_env_add;
 };
 
 }