When using the multview / docking feature of dear imgui (availible on the main repo in the docking branch) dear imgui will use desktop relative coordinates instead of window relative. This causing the rendering to get "offseted" if not handled correctly.
Before this change the rendering matrix wasn't used at all and this has now been changed in the vertex shader.
Notice this change is fully backwards compatible with the relative windows as well as the upper left corner will always be set to 0,0 in that case thus this change will work with both versions.
I also added some changes to skip rendering if not needed (based on the the other backend implementations for dear imgui such as the OpenGL one)
c9bf791eaa