Support Screenspace raycasting for cameras rendering to a texture

[dtaralla]

Hi!
Since the render-to-texture cameras feature has been merged in bevy's main branch, here is a simple change to allow supporting 1st pass cameras rendering to an Image.

Maybe there is another cleaner way to do this. I find it a bit intrusive to have to gather Res<Assets<Images>> along Res<Windows> just to be able to support both cases.
With generics? Unfortunately I'm not fluent enough with Rust generics to find a way to do it (I wanted to make get_camera_rendering_size a generic with "template argument" from C++ with RenderTarget enum values, but it seems we can't do that yet in Rust).

[dtaralla]

Thanks for adding support for this with your Bevy 0.7 commit! 🙏

[aevyrie]

Apologies for neglecting this PR. Has the latest release worked for your use case?

[dtaralla]

Yep, thanks!