Text: query the actual format in GlyphCacheGL::processedImage().

Right now it didn't really matter as the only subclass making use of
this was DistanceFieldGlyphCacheGL in which the processed format was
always R8Unorm, nevertheless it's better this way, and makes it work
with 16-bit output and such that might get added in the future.

src/Magnum/Text/GlyphCacheGL.cpp

@@ -192,7 +192,7 @@ void GlyphCacheGL::doSetProcessedImage(const Vector2i& offset, const ImageView2D
 
 #ifndef MAGNUM_TARGET_GLES
 Image3D GlyphCacheGL::doProcessedImage() {
-    Image2D out = static_cast<State&>(*_state).texture.image(0, PixelFormat::R8Unorm);
+    Image2D out = static_cast<State&>(*_state).texture.image(0, processedFormat());
     return Image3D{out.format(), {out.size(), 1}, out.release()};
 }
 #endif