Add support for WebP image format

[brunopicinin]

Description

Added support for WebP image format. Pretty much every browser, except for IE, supports this format.

No major changes in code required, since the image decoding is handled by the native Image object.

Type of change

• New feature (non-breaking change which adds functionality)

How has this change been tested, please provide a testcase or example of how you tested the change?

WebP image:

• Dragged a webp image to the canvas
• Created some polygons on top of it
• Saved annotated image to validate export

PNG/JPEG/JPG images:

• Dragged other supported images to the canvas to validate that other images still work

Other files:

• Dragged a couple of text files to the canvas to make sure that non-image files throw an alert

[capjamesg]

Looks good.