shiny/driver/gldriver: update the open gl context after making it current

This seems necessary (at least on Mojave). Otherwise, the window seems to have the wrong viewport until it is moved or resized.

Change-Id: I0efd372e875e37056974a9d1ae99ba47896f69d5
GitHub-Last-Rev: bc6b239087
GitHub-Pull-Request: golang/exp#5
Reviewed-on: https://go-review.googlesource.com/c/154461
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>

shiny/driver/gldriver/cocoa.m

@@ -34,6 +34,7 @@ enum
 void makeCurrentContext(uintptr_t context) {
 	NSOpenGLContext* ctx = (NSOpenGLContext*)context;
 	[ctx makeCurrentContext];
+	[ctx update];
 }
 
 void flushContext(uintptr_t context) {