This patch also makes Optional public to end users.

This includes Carbon support

This restores the functionality of JUCE_COREGRAPHICS_RENDER_WITH_MULTIPLE_PAINT_CALLS.
Using this preprocessor flag may avoid Core Graphics rendering much larger regions than
necessary, but the small regions that are rendered will likely be rendered slower.
Whether using this flag improves or degrades the performance of your rendering overall
will be specific to each application.

Previously enabling JUCE_COREGRAPHICS_RENDER_WITH_MULTIPLE_PAINT_CALLS was ineffective
from versions of macOS around 10.13, but enabling it didn't have any negative impact on
performance. Now enabling JUCE_COREGRAPHICS_RENDER_WITH_MULTIPLE_PAINT_CALLS may result
in slower rendering.

When calling NullCheckedInvocation::invoke with a capture-less lambda,
GCC 9.3 determines that the expression
    std::declval<TheLambda>() != nullptr
is well-formed, and uses the version of invoke containing a nullptr
check. However, the compiler is also able to determine that this
expression can never be false, and emits a warning.