Add $meters_per_pixel keyword for filters and JS functions

[matteblair]

• Add the $meters_per_pixel keyword, determine its value from the current zoom in StyleContext, and make it accessible to filter expressions and JS functions.
• Clean up the logic for keywords in StyleContext - setting arbitrary keywords is no longer supported.
• Updated tests for changes to StyleContext and added a case using $meters_per_pixel in a JS function.

Resolves #2138

[westnordost]

Very cool! :-D

Could it be that with this change, you also changed unintentionally or not the $zoom keyword from an int value to a float value?

[matteblair]

The variable that provides the value of $zoom was changed from an int to a double to reduce casting, but it still holds an integer value (because filters and styling functions are only applied at integer zoom levels).

[westnordost]

Oh, that means that meters_per_pixel only changes per integer zoom level?

…

On 9 February 2020 08:05:22 CET, Matt Blair ***@***.***> wrote: The variable that provides the value of `$zoom` was changed from an `int` to a `double` to reduce casting, but it still holds an integer value (because filters and styling functions are only applied at integer zoom levels).

[matteblair]

Yes. Filters and styling functions are evaluated on features once, using the zoom level of the tile that contains them.

[westnordost]

Oh, too bad, so the use case I had in mind for this will not work (#1121)