Add option to skip the clearing of textures

[cabanier]

The WebXR layers spec defines that the textures that are returned from the binding are always cleared to opaque black.
This feature is hard to implement without incurring GPU overhead especially when dealing with multiple layers.

Could we introduce an option to skip this behavior and leave it up to the experience to clear its buffers?

/agenda Add option to skip the clearing of textures

[DRx3D]

There was a request that it might be better to name the flag so that the default (FALSE) implies what is desired. The initial proposal was for 'clearOnAccess' with the default value of TRUE. A field with a default of FALSE could be called 'alwaysClear'.

[AdaRoseCannon]

Removing label, because it was discussed at the last meeting, if this is in error or more discussion is needed please reuse '/agenda'

[cabanier]

@toji I looked at various idl interfaces in chromium and there are many that initialize dictionary values to true.
I prefer to keep it as "true" because it's less confusing. (False would require a double negative)

[cabanier]

/agenda can we keep clearOnAccess?