Browser Warning due to missing `willReadFrequently` on heatmap

[bebeal]

To reproduce, make a heatmap, open up the console and see:

plotly.js:114952 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
getPixels @ plotly.js:114952
vectorizeText @ plotly.js:114956
createText @ plotly.js:114946
tryVectorizeText @ plotly.js:113624
addItem @ plotly.js:113617
proto.update @ plotly.js:113619
createTextSprites @ plotly.js:113624
proto.update @ plotly.js:113562
createAxes @ plotly.js:113595
createScene @ plotly.js:113915
__webpack_modules__.33539.proto.tryCreatePlot @ plotly.js:52996
__webpack_modules__.33539.proto.initializeGLPlot @ plotly.js:53036
Scene @ plotly.js:52946
plot @ plotly.js:52053
__webpack_modules__.61549.exports.drawData @ plotly.js:36679
__webpack_modules__.71828.lib.syncOrAsync @ plotly.js:26541
continueAsync @ plotly.js:26537
Promise.then (async)
__webpack_modules__.71828.lib.syncOrAsync @ plotly.js:26543
_doPlot @ plotly.js:31810
newPlot @ plotly.js:31969
react @ plotly.js:33884
eval @ factory.js:89
Promise.then (async)
updatePlotly @ factory.js:79
componentDidMount @ factory.js:119
reappearLayoutEffects @ react-dom.development.js:23052
recursivelyTraverseReappearLayoutEffects @ react-dom.development.js:23156
reappearLayoutEffects @ react-dom.development.js:23038
...

This line needs updating:

plotly.js/src/traces/heatmap/plot.js

Line 165 in 915a52b

var context = canvas.getContext('2d');

similar to #6084

[bebeal]

Resolved with pr: #6741