For blob image exports, allow use of `canvas.toBlob()` where available

[maxkfranz]

Issue type

Feature request

Description of new feature

When using image export functions like cy.png(), it would be nice to use canvas.toBlob() if the browser has the function available.

• Chrome (50+), though mobile may be limited
• Firefox (25+)
• Safari, though mobile may be limited
• Edge (possibly supported, partially supported by canvas.msToBlob())
• IE (partially supported by canvas.msToBlob()

Note that this would require new API, as canvas.toBlob() is asynchronous, whereas cy.png() and cy.jpg() are synchronous: cy.png({ output: 'blob-promise' })

Motivation for new feature

These items would be nice, but it seems that the browsers don't actually provide these benefits in practice:

• Faster speed for downloads than using a base 64 export as a intermediary format.
• Bigger exported images (file size). Base 64 URIs have different size limitations on different browsers.

These items are confirmed benefits:

• Non-blocking blob generation. This allows for keeping the UI responsive while the download is happening in the background.