Prevents the following compiler warnings:

heif_context.cc: In function 'std::shared_ptr<heif::HeifPixelImage> create_alpha_image_from_image_alpha_channel(std::shared_ptr<heif::HeifPixelImage>)':

heif_context.cc:1623:48: warning: conversion from 'int' to 'uint16_t' {aka 'short unsigned int'} may change value [-Wconversion]
 1623 |   alpha_image->fill_new_plane(heif_channel_Cb, half_range, chroma_width, chroma_height, bpp);
      |                                                ^~~~~~~~~~

heif_context.cc:1624:48: warning: conversion from 'int' to 'uint16_t' {aka 'short unsigned int'} may change value [-Wconversion]
 1624 |   alpha_image->fill_new_plane(heif_channel_Cr, half_range, chroma_width, chroma_height, bpp);
      |                                                ^~~~~~~~~~