Reduce number of blits required in stereo 3D by tracking current eye...

[cmbruns]

...to improve efficiency. Each postprocessing pass will now begin its work on whichever eye the previous pass finished on.

[coelckers]

This needs to be checked by dpJudas, especially if this has any bearing on porting the code to Vulkan.

[cmbruns]

@dpjudas Please note that this change is closely related to #760 which is already merged into master. In that previous pull request, I replaced most of the framebuffer swaps used for stereo 3D, with blits, because I felt it was easier to reason about; especially in the context of the framebuffer swapping used for postprocessing. This pull request is a follow on to that one, and reduces the total number of blits, by reducing the number of eye context switches. Thus each loop-over-the-eyes loop begins with the eye that the previous one finished on. The first such loop, the regular 3D scene render, always begins at eye number zero, in case the stereo mode changed since the previous frame.

[dpjudas]

This is probably okay to merge as-is, given the other PR is already merged in.

There are a lot of subtle differences between OpenGL and Vulkan framebuffers. Most important one being that the vulkan ones are tied to a specific render pass. The concept of renderbuffers also no longer exists in Vulkan. The Vulkan vkCmdBlitImage command is also far more restrictive than its OpenGL counterpart and requires image transition operations which could negatively affect performance.

For all those reasons I won't be trying to move more of FGLRenderBuffers to the hwrenderer layer as the differences are just too great.