Steps to repro:

1. Open attached project and 'test' scene on OSX;
2. Open Frame Debugger (Window -> Frame Debugger) and click 'Enable';
3. Select 'Draw Mesh' in Camera.Render - Drawing - Camera.RenderSkybox section of FrameDebugger window;

4. Notice that assertion:
'Assertion failed on expression: 'srcAttach < m_CurrentFramebuffer.colorCount && "We should always resolve only current RT"' '
is getting spammed in the console window.

5. Try to switch frame Debugger frames either by moving frame slider or by clicking arrow buttons inside of it.

Actual result:
Frame Debugger doesn't render overlaying draw calls (see attached gif video).

Regression introduced in Unity 5.6.

Expected result:
- Frame debugger should always switch frames and show overlaying draw calls;
- Assertions should not be thrown when operating Frame Debugger.

Notes:
- Reproducible in 5.6.0a3;
- Not reproducible in 5.5.0f2, 5.4.3f1;
- Reproducible only on OSX 10.12.1;
- Not reproducible on Windows 10;
- Reproducible only in Forward with Shadows turned off;
- Enabling shadows on Directional fixes the issue.