Improve render-to-texture framebuffer selection

Specifically:
 * Prioritize more recently rendered to matching framebuffers.
 * Keep a render source alive if it's being used, even if not rendered to (fixes Mana Khemia blurred dialog scenes.)
 * Use an existing framebuffer right away, instead of only after the next `SetRenderFramebuffer()`.

Might improve other framebuffer issues, might really break them.... Might make AttachFramebufferInvalid() work better also.

-[Unknown]