shiny/driver/gldriver: fix 2 warnings on macOS 10.14
shiny/driver/gldriver: fix 2 warnings on macOS 10.14

This CL iterates upon CL 171033 and resolves two remaining C compiler
warnings when building on macOS 10.14:

	# golang.org/x/exp/shiny/driver/gldriver
	cocoa.m:63:39: warning: 'NSOpenGLCPSwapInterval' is deprecated: first deprecated in macOS 10.14 [-Wdeprecated-declarations]
	/Applications/Xcode.app/.../NSOpenGL.h:281:39: note: 'NSOpenGLCPSwapInterval' has been explicitly marked deprecated here
	cocoa.m:72:1: warning: method possibly missing a [super prepareOpenGL] call [-Wobjc-missing-super-calls]

NSOpenGLContextParameterSwapInterval is the less-deprecated version of
NSOpenGLCPSwapInterval and respects the GL_SILENCE_DEPRECATION define.
However, we can't use it because it's available only in macOS 10.12+,
but Go currently supports older macOS versions (e.g., Go 1.12 requires
macOS 10.10+). Use a targeted #pragma clang diagnostic ignored instead.

Add a [super prepareOpenGL] call to ScreenGLView.prepareOpenGL method.
I can't quite tell if it was omitted intentionally, but based on its
documentation¹, it seems reasonable to have it. I didn't notice problems
in local testing. If it turns out it shouldn't be there, then it can be
removed in a way that doesn't generate a compiler warning.

¹ https://developer.apple.com/documentation/appkit/nsopenglview/1414940-prepareopengl

Updates golang/go#11811

Change-Id: I10b6522a7af64eadeb6586fad44ef4c1ece40ab2
Reviewed-on: https://go-review.googlesource.com/c/exp/+/171034
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/exp,golang/exp,golang/exp