RMT Buffer Allocation Fix (Thread-Safe) for Issue #375

[teknynja]

NOTE: This is a thread-safe version of pull-request #392: It is essentially the same code with a mutex added to protect the code from multiple threads.

This pull request addresses issue #375 [pixel.show() crash with more than 73 pixel on ESP32s3]

After reviewing the code and also getting some helpful feedback from the Espressif Forums (https://www.esp32.com/viewtopic.php?f=13&t=40270) and @robertlipe it was determined that the code in esp.c for handling the RMT item buffers when using the IDF v5 framework was allocating too much space on the stack when using more than 70ish pixels.

This pull request addresses that issue by allocating the RMT buffers from the heap instead. It will attempt to allocate a single block of memory to accommodate the largest configured instance (sharing the buffer between instances is fine, as the buffer is completely populated each each time the espShow() method is called).

I also took the time to improve the channel allocation management, previously the RMT channels were initialized on each call to espShow(), now the RMT channels are only de-initialized and re-initialized whenever the output pin is changed.

Finally, I was concerned about problems that may be caused by allocating large buffers on the heap without giving the user any way to free that memory, so the code allows a user to free that memory (and also release the RMT channels) by setting the number of pixels to zero using .updateLength(0) and then calling .show(). This will de-allocate the heap memory used for the RMT buffers and release the RMT channels held by driver. They will automatically be re-allocated if needed when setting the number of pixels back to a non-zero value.

[teknynja]

Updated code to move espShow()'s mutex initialization to the Adafruit_NeoPixel constructor, allowing users to avoid a race condition.

[robertlipe]

I like this. Thank you, teknynja.

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