Fix starting up in non-native fullscreen resulting in black screen

[ychin]

This fixes issues related to starting up in fullscreen mode (by setting set fullscreen in gvimrc).

Fix black screen when starting in non-native fullscreen:

• Fix set fullscreen goes to black after update 8.1.560 #804
• Non-native fullscreen has an old behavior where it sets CGLayer mode on core text renderer to address rendering artifacts in pre-Mojave, and we have an insufficient block against it (by checking cgContext). In Mojave, we use buffered draws instead and prevent setting CGLayer by checking cgContext, but it is not yet initialized yet in this situation (since it's lazily initialized). Block against cgBufferDrawEnabled instead.
• The CGLayer renderer is really deprecated at this point so ideally we could clean up the code a little bit in the future.

Setting MMUseCGLayerAlways no longer results in black screen:

• Fix All text invisible after update to 8.1.560 on Mojave #801
• CGLayer renderer is basically deprecated doesn't actually work in Mojave. Make sure setting MMUseCGLayerAlways won't trigger it if we are already using buffered draw (which is automatically the case in Mojave).

Fix starting up in fullscreen resulting in a small Vim window:

• When starting MacVim in fullscreen mode, the presentation is delayed until the window is made. need to make sure to set shouldResizeVimView in this code path too. Before the guioption-k fixes, there were a lot of random calls to recalculate sizes which was why this happened implicitly. Now need to make sure we do the right thing and call it.