New VTEnc API

[vteromero]

This is a redesign of the VTEnc API.

Main changes:

• Removes VtencEncoder and VtencDecoder structures, which are replaced with the opaque structure vtenc.
• Removes vtenc_encoder_init and vtenc_decoder_init functions.
• Adds vtenc_create and vtenc_destroy functions to create and destroy the handler.
• Adds vtenc_config function, which allows you to set the encoding parameters after the handler creation. This function also opens the possibility of setting other configuration values in the future.
• Adds vtenc_encoded_size function to get the size of the encoded output after calling a vtenc_encode* function.
• Updates signatures of vtenc_encode* and vtenc_decode* functions to take a vtenc handler and return an integer with a result code.