reduce buffer scale to 0.5 for iOS safari < 10 (fixes #1714)

[ngokevin]

Description:

Workaround for Webkit bug on iOS Safari < 10.

Before

Upon Entering VR from landscape without orientation change:

After

Stereo now sized correctly, at the cost of decreased resolution. With the version check, this should automatically be addressed once people upgrade to iOS 10 as the Webkit patch rolls out.

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[dmarcos]

I have to try this tomorrow on my iphone 6 Plus that has devicePixelRatio === 3

[dmarcos]

The canvas size gets fixed when turning the phone to portrait and back to landscape. Is avoiding that glitch more important than keeping the high resolution?

[ngokevin]

I think so, I don't think it's acceptable that people (not developers) have to know to rotate their phone back and forth. Many would think it was just broken and leave. With the version check, trying to have it automatically be addressed once iOS 10 starts rolling out.

I'll change it to 1 / window.devicePixelRatio then for iPhone Plus.

[dmarcos]

Yes, I confirm we need 1 / window.devicePixelRatio to make it work on the iphone6 Plus