When running on macOS 10.14, both master and libreoffice-6-1 (at least) started
to fail CppunitTest_vcl_bitmap_render_test with

> /Users/stephan/Software/lo2/core/vcl/qa/cppunit/bitmaprender/BitmapRenderTest.cxx:90: Assertion
> Test name: BitmapRenderTest::testTdf113918
> equality assertion failed
> - Expected: mcBlueOrIndex: 255, mcGreen: 255, mcRed: 255, mcAlpha: 0
> - Actual  : mcBlueOrIndex: 1, mcGreen: 112, mcRed: 15, mcAlpha: 0

in --enable-debug/--enable-dbgutil builds.

I tracked that down to the OSL_DEBUG_LEVEL>0--only call to
Scheduler::ProcessEventsToIdle in test::BootstrapFixture::setUp
(test/source/bootstrapfixture.cxx).  If that call is also included in non-debug
(i.e., OSL_DEBUG_LEVEL=0) builds, or if it is removed from all builds and
instead an unsuspecting

  [NSApp nextEventMatchingMask: 0 untilDate: nil inMode: NSDefaultRunLoopMode dequeue: NO];

is added directly before the call to

  pVDev->SetOutputSizePixel(Size(2480, 3508));

at the start of BitmapRenderTest::testTdf113918
(vcl/qa/cppunit/bitmaprender/BitmapRenderTest.cxx:76), the test always fails.

What happens is that, after such a nextEventMatchingMask call done, the call to
NSGraphicsContext graphicsContextWithWindow in AquaSalVirtualDevice::SetSize (as
called from the SetOutputSizePixel call in the test) returns nil, for unclear
reasons.

Until those underlying reasons are understood, change the code of SetSize to
"fall back to a bitmap context" also in case the graphicsContextWithWindow call
returned nil.

Change-Id: I604ebd2c444b58610e4ed73ec70af87b0a08e864
Reviewed-on: https://gerrit.libreoffice.org/61519
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>