Replace Jimp with canvas for loading images

For the following reasons:
1. Size - Jimp is a large package, making it less suitable for use in the browser. Most of the functions aren't even used by this library.
2. Speed - According to https://github.com/ivanoff/images-manipulation-performance, `node-canvas` is around 9.3x faster than Jimp.
3. `Image` class - Their `Image` class did not support easily converting between colour formats (RGB vs. RGBA vs L), which is needed for mask images (for example).

The main downside is that when resizing, the interpolation method isn't able to be specified (e.g., CLIP using bicubic). We aim to fix this in the future.

Author: xenova