I was wondering if WebGL is considered stable enough to be enabled by default. It makes some PDFs render much faster and it has been off by default for a while now, so we have had quite some time to test it.

If we decide to enable WebGL by default, we should not forget to update https://github.com/mozilla/pdf.js/blob/master/extensions/chromium/preferences_schema.json#L39 as well.

/cc @yurydelendik 
/cc @Snuffleupagus (because IIRC you were testing WebGL too)

"\
  precision mediump float;                                      \
                                                                \
  uniform vec4 u_backdrop;                                      \
  uniform int u_subtype;                                        \
  uniform sampler2D u_image;                                    \
  uniform sampler2D u_mask;                                     \
                                                                \
  varying vec2 v_texCoord;                                      \
                                                                \
  void main() {                                                 \
    vec4 imageColor = texture2D(u_image, v_texCoord);           \
    vec4 maskColor = texture2D(u_mask, v_texCoord);             \
    if (u_backdrop.a > 0.0) {                                   \
      maskColor.rgb = maskColor.rgb * maskColor.a +             \
                      u_backdrop.rgb * (1.0 - maskColor.a);     \
    }                                                           \
    float lum;                                                  \
    if (u_subtype == 0) {                                       \
      lum = maskColor.a;                                        \
    } else {                                                    \
      lum = maskColor.r * 0.3 + maskColor.g * 0.59 +            \
            maskColor.b * 0.11;                                 \
    }                                                           \
    imageColor.a *= lum;                                        \
    imageColor.rgb *= imageColor.a;                             \
    gl_FragColor = imageColor;                                  \
  }                                                             ";
  let smaskCache = null;
  function initSmaskGL() {