Don't signal fence and flush in renderer, let user control it

[hakolao]

hmmm... might be even better to just return the command buffer...

[RA3236]

There are a couple of other times in the code where a command buffer is submitted and executed mid-frame (notably Renderer::update_textures()). What could probably work better is having a single secondary command buffer that is passed to each function and relying on the user to wait() when necessary, but I would reckon that #52 should be merged once vulkano is updated before doing major changes like that.

[hakolao]

True