Discourage developers from adding canvas sources to style JSON

[1ec5]

The canvas source type documentation should state that canvas sources are only to be used at runtime in JavaScript code, never in style JSON. This source type is inherently platform-specific: some platforms like iOS have no concept of a rendering surface associated with a unique identifier. A platform-specific source type has no place in a file format that’s intended to be portable across platforms.

Ideally, canvas would be omitted entirely from the style specification. However, GL JS uses the style specification to document its runtime styling API, so that isn’t an option right now.

/cc @anandthakker

[jfirebaugh]

1c719f1