imprv: Windows: Package pixbuf loaders for avif and webp

Can't test this myself, please someone do it for me. :)

Some images for testing:

• https://raw.githubusercontent.com/AOMediaCodec/av1-avif/master/testFiles/Xiph/quebec_3layer_op2.avif
• https://upload.wikimedia.org/wikipedia/commons/a/a1/Johnrogershousemay2020.webp