Update code to work with new version of ImGui.

UpdateImGuiTexture now handles creating font textures and modifying them.

Source/Core/VideoCommon/AbstractTexture.cpp

@@ -20,6 +20,11 @@ AbstractTexture::AbstractTexture(const TextureConfig& c) : m_config(c)
 {
 }
 
+AbstractTexture::operator ImTextureRef() const
+{
+  return ImTextureRef(reinterpret_cast<AbstractTexture::imgui_texture_id>(this));
+}
+
 void AbstractTexture::FinishedRendering()
 {
 }