Default to WebGL "high performance" mode

[bcamper]

Defaults to using high-performance for WebGL's powerPreference setting. This will ask for use of the discrete GPU where possible. The user can override as with other GL context options:

const layer = Tangram.leafletLayer({ scene, { webGLContextOptions: { powerPreference: 'low-power' } } });

[bcamper]

Discussed in chat and further with @meetar.