After losing the window focus, Render Textures stopped rendering.

To reproduce this:
- download the minimal project in the comments
- notice the game tab shows blue
- Play the "repro" scene
- notice the game tab now shows black
- minimize and go back to unity again - game tab now shows all white

Expected:
- game tab is solid black color after minimise
Actual:
- game tab solid white color after minimise

Reproduced on Metal, OpenGL, DX and Vulkan
Reproduced on Windows and MacOS
Cannot reproduce in 2019.2.4f1
Reproducible in 2019.3.0a1, 2019.3.0a2, 2019.3.0a8, 2019.3.0b1 and 2020.1.0a2

Workaround:
Add the following code block before the first SubShader block in both Light2D shaders "LightBlender.shader" and "NormalMappedLight.shader"

Properties{
_MainTex("_MainTex", 2D) = "black" {}
}