Validate maximum number of data-driven paint properties per layer

Nobody has claimed this yet.

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).

No source file or test is named in the issue. Start by locating the style-validation entry point and the per-layer shader attribute definitions; verify the computed limit for each layer type and add validation coverage showing that styles exceeding the limit are rejected.

Written by the indexing model from the issue text.