Optimize image rasterization with willReadFrequently hints

[mourner]

When using getImageData (e.g. for DEM tiles), we reuse the same canvas object and then read it repeatedly for different images. Recent versions of Chrome started throwing this warning in the console:

Canvas2D: Multiple readback operations using getImageData are faster with the willReadFrequently attribute set to true. See: https://html.spec.whatwg.org/multipage/canvas.html#concept-canvas-will-read-frequently

This PR adds willReadFrequently hints in two places where it's needed to make sure that getting data out of loaded images is as fast as possible.

Launch Checklist

• briefly describe the changes in this PR
• post benchmark scores
• manually test the debug page
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Improve performance of loading terrain data</changelog>

[stepankuzmin]

👍