cam: Improve memory allocation
cam: Improve memory allocation

 * realloc the dma descriptors and buffers, so we can change the image size
 * NULL out the camera pointer after deinit, so deinit is idempotent

adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython