The spec currently says

GPUImageCopyExternalImage has the following members:

source, of type GPUImageCopyExternalImageSource
The source of the image copy. The copy source data is captured at the moment that copyExternalImageToTexture() is issued. Source size is defined by source type, given by this table:

It's not clear what happens if you have a WebGL canvas where canvas.width > gl.drawingBufferWidth etc...