webgl: support iOS devices

[fs-eire]

This change fixes the webgl failure on iOS devices.

• [misc] enable check in debug mode
• allow half_float textures
  • support half_float textures rendering
  • support detections of float rendering availability
  • support detections of float texture data downloading
  • update test runner to support half float
• [bugfix] update condition for forceReadUint8
  • check by flag floatDownloadEnabled instead of browser name
• [bugfix] fix isNaN in iOS12
• test: update whitelist for iOS
• [misc] use high precision sampler2D by default
• [bugfix] webgl: stop using window.WebGLRenderingContext
  • fix bug on Safari 10.x (where window.WebGLRenderingContext is not available)
• [misc] test: update karma config for BrowserStack
  • fix BrowserStack tests caused by incorrect browser version
• [misc] test: optimize randomArray in conv test
  • fix BrowserStack tests caused by big number ( >> HALF_FLOAT_MAX )
  • make input values smaller ( in range 0~1)