WebP sub-project searches for OpenGL, and sets the OpenGL variables to NOT_FOUND.
Make sure all OpenGL variables are updated when OpenGL dependency is set manually.

Add function to get the WebP library version.

Add LoadAnimation function to get all frame images, background colour and duration.

Use WebPAnimDecoder to decode frames other than the default (-1) frame.
This ensures all returned frames will have the same image size.
WebPDemuxGetFrame can return frames smaller than the full image size, but there is no way to get the offset of the frame.

Reduce code duplication when creating rgba image.

Add enum to store image format (undefined/mixed, lossy or lossless).
Add lossless and lossy quality options to wxImage for saving WebP.

Add wxBITMAP_TYPE_WEBP_RESOURCE because all other image handlers have this too.

Improve high-dpi support.

Run upmake, bakefile_gen, makeprojects.py, update-setup-h, autogen.