fix resolution in GearVR/Oculus browser

[gmarinov]

fix #2078

background: devicePixelRatio when isPresenting is 1 and canvas sizing must be determined from the actual renderWidth and renderHeight parameters for each eye.

for the GearVR Samsung S6 renderWidth and renderHeight equal 1024, but the display is 2560x1440, so some upscaling is forced.

I experimented with calculating devicePixelRatio instead in order to achieve higher quality image, but got inconsistent results across the range VR/iOS/Android-cardboard.

tested on:

• macOS: desktop (Safari),
• iOS: magic window & polyfilled VR
• Android: magic window, polyfilled VR, GearVR/Oculus browser

[esaruoho]

Gear VR Oculus Browser, am seeing non-pixelation