encode() Does Not Respect ColorEncoding

[namse]

When ColorEncoding is set to luma, num_channels should be 1. However, in the current code, if luma is used, the pixel_format() of EncoderFrame always returns num_channels as 3. This should be 1.

[inflation]

The default EncoderFrame is RGB no matter the encoder's settings. You can customize it to your needs.

This is because you don't need to keep the input pixel format and the basic info of the encoder to be precisely the same. You could provide an RGB image buffer to be encoded as greyscale or vice versa.