vout: split RenderPicture()

The first part of RenderPicture() filters the input picture. This process is independent of the display.

The second part renders SPU then calls prepare()/display() on the vout display module. This depends on display properties (in particular its dimensions and ratio).

In order to be able to "restart" the second part on resize, extract it to a separate function.

This is initially a preparation commit from !324. Even if its purpose is related to !324, I think the change itself could be merged beforehand.