How to reproduce:
1. Download attached project file attached by QA and open "Basic" scene
2. Click on SoftShadowProjector -> SoftShadowProjector in Hierarchy Window
3. Press Play

Expected result: Blurring, generating mipmaps in OnPostRender function should work in Edit Mode
Actual result: Blurring, generating mipmaps in OnPostRender function works when the scene is running, but it doesn't work in Edit Mode

Reproduced with: 5.6.0f1, 5.6.2p3, 2017.1.0b7, 2017.1.0f3, 2017.2.0b4, 2017.3.0a1
Not reproduce with: 5.5.2f1, 2017.1.0a1, 2017.1.0b6

Regression: 5.6.0f1 - 5.6.2p3 (Repro), 2017.1.0b7 - 2017.3.0a1 (Repro)

Note: Tested on these API's: DirectX9, DirectX11, DirectX12, Vulkan;