Note: This precedes another CL that needs this change.

DebugAnnotator uses a global variable.  The test runner doesn't change
state between testing different back-ends.  This works-around the
problem by setting the global variable when the context is switched.

Because the GL back-end doesn't have its own DebugAnnotator sub-class,
add a Display* to DisplayImpl::makeCurrent(), so that
DisplayGL::makeCurrent() can install the front-end-Display's
DebugAnnotator.

Note: the Vulkan back-end gets this fix even though the new
DebugAnnotatorVk class will be added in a follow-on CL.

Bug: b/162068318
Bug: b/169243237
Bug: angleproject:5121
Change-Id: If08626a5310f9b4e3210e1a897a6886248e4d8ac
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2451423Reviewed-by: Ian Elliott <ianelliott@google.com>
Commit-Queue: Ian Elliott <ianelliott@google.com>