D2D repaints are always driven by a vblank timer, and D2D presentation
can't happen any faster than the vblank callbacks, so I think it's safe
to remove the swapchain machinery and to rely entirely on the vblank
callbacks instead.
This makes Direct2DMetrics and current frameId accessible to implementation subclasses.
It also replaces JUCE_WRITE_TRACE_LOG with JUCE_WRITE_TRACE_LOG_VA as intended in original implementation.
Co-authored-by: Matt Gonzalez <matt@echoaudio.com>