Validate maximum number of data-driven paint properties per layer

[jfirebaugh]

The maximum number of properties that can be data-driven in a single layer simultaneously is effectively limited by GL_MAX_VERTEX_ATTRIBS, which can be as low as 8. As we grow the number of data-driven properties, it becomes more likely that a style author could go over this limit unknowingly, test on devices that support GL_MAX_VERTEX_ATTRIBS > 8, see nothing wrong, and then deploy and have the style break on lower-end devices.

We should introduce style validation to prevent this. The validation rule should enforce a limit on the number of data-driven paint properties used per layer. The limit varies by layer type, and is defined by (8 - maximum number of non-paint-property attributes required by any shader used for that layer type).

[jfirebaugh]

This is, unfortunately, a validation that may become stricter over time, if we introduce features that require adding new non-paint-property attributes to a shader. Not sure there's any solution for that other than to avoid such features.

[anandthakker]

Should this be something that we flag in Studio as well? @mapbox/studio

[jfirebaugh]

Current limits: circle: 7, fill-extrusion: 6, fill: 7, heatmap: 7, line: 6, symbol: 4. For symbol layers, the limit is independent in text-* properties and icon-* properties. That is, you can make up to 4 text-* properties data driven, and simultaneously, up to 4 icon-* properties.

These numbers are subject to change, but effectively, given the current set of data-driven properties, the only way to hit this limit is to make all the icon-* paint properties or all the text-* paint properties data-driven simultaneously.