Add resumeDMAoutput() to resume DMA after stopDMAoutput()

[rafma0]

As discussed in #656.

stopDMAoutput() halts the DMA engine but, until now, there was no public way to start it again short of rebooting. This PR adds a resumeDMAoutput() method that calls the existing dma_bus.dma_transfer_start() (the same call begin() already uses to start output), so a stopped panel can be brought back without a reboot.

Use cases

• Reduce power draw / 'turn off' the panel when not in use, then resume on demand.
• Quiet the panel's RF emissions during sensitive WiFi transfers (stop, fetch, resume), which noticeably improved my fetch reliability.

Notes

• stopDMAoutput() blanks the framebuffer content (brightness is preserved via resetbuffers()), so callers should redraw after resuming.
• Doc comment on stopDMAoutput() updated (it previously said the screen stays black until reboot, which is no longer the case).