setup canvas on first capture screenshot.js

[arpu]

Description:
The default Screenshot Component always creates a shader and canvas on init()
this PR change it on first usage of screenshot capture.

Changes proposed:
-set canvas on first capture usage

[dmarcos]

Thank you!

[mrxz]

@arpu This PR causes a regression when people use the screenshot component in the following way:

document.querySelector('a-scene').components.screenshot.getCanvas('perspective');

The getCanvas is part of its public API and documented in case the user doesn't want to automatically download the file, but do something else with it. See https://aframe.io/docs/1.5.0/components/screenshot.html#methods