stbi is a thin wrapper around stb's stb_image.h, which in turn
is "a public domain image loader" for popular graphics image file formats.
By default, stbi.load and stbi.load_from_memory convert images to 4-channel RGBA pixel data.
Image.nr_channels reflects that in-memory layout, while Image.original_nr_channels keeps the
channel count from the source file as metadata.
When uploading to sokol.gfx, size the upload from width * height * nr_channels.
If you load with desired_channels: 0, convert the data yourself or use a matching pixel format
before creating a texture.
Also call gfx.setup(...) before gfx.make_image(...) and gfx.make_sampler(...).