Rename GLRenderTarget to GLFramebuffer
GLRenderTarget doesn't provide a generic abstraction for framebuffer objects, so let's call GLRenderTarget what it is - a framebuffer.
Renaming the GLRenderTarget class allows us to use the term "render target" which abstracts fbos or shm images without creating confusion.
Extracted from !2229 (merged). Affects too many files so there's high risk for merge conflicts.