OvGame isn't rendering properly

[adriengivry]

Description
Since the renderer rework (#277), it seems like OvGame isn't rendering the game to the backbuffer properly. The new renderer relies on an explicit framebuffer to work, which OvGame doesn't have as it renders to the backbuffer directly.

To Reproduce
Steps to reproduce the behavior:

1. Open Overload
2. Make a temporary build
3. See error

Expected behavior
OvGame should also use a framebuffer, and finally blit this framebuffer onto the backbuffer at the end of a frame.

Screenshots
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