SCREEN_TEXTURE does not correctly contain the skybox in the first frame

The Lancelot test case 'transmission' highlights a problem where the output from the View3D is different once the window gets further updates, e.g. due to resizing.

A capture shows why this is: the contents of the screen texture is simply different. On the left, the capture is from (hopefully) the first frame. On the right, the second capture is after resizing the window. Shown there is the screen texture. Note how it does not have the light probe texture used as the skybox on the left.