Previously, we were using the window's top-left position to determine
the scale to use for the OpenGLContext. However, on macOS the
backingScaleFactor of the window is not strictly related to the top-left
corner of the window, so the OpenGL view's scale could end up differing
from the backing scale factor when slowly moving a window between
displays with different backing scale factors.

On macOS, we now use the backing scale factor of the window's screen (as
maintained by AppKit), rather than trying to work out the correct
display and scale ourselves.

When FileChooser instances were created, launched, and hidden all inside
the same event callback on macOS 12.0.1, the chooser dialog sometimes
remained open. This could cause problems including crashes, as closing
the dialog would attempt to call a completion handler block referencing
an already-deleted FileChooser::Native instance.

Opening the chooser panel later on the message thread seems to resolve
the issue.

This commit reverts c34f13a02d

In stripped plugin builds, the repostCurrentNSEvent function was still
public. There doesn't seem to be a good reason for this, as no host
should need to call the function.

It looks like the shared code component of a Projucer-generated plugin
project was briefly built as a framework that was shared by the
different plugin wrappers. In this scenario, the framework would need to
make repostCurrentNSEvent public so that the wrappers could locate the
function. However, now that the shared code target is built as a static
library, standard external linkage should be sufficient for the symbol
to be located from the wrapper's TUs.

This fixes an issue on iOS platforms where the version number string is
prefixed with "OpenGL ES " despite the Khronos docs for OpenGL ES
specifying that "The GL_VERSION and GL_SHADING_LANGUAGE_VERSION strings
begin with a version number".