Make style properties null-resettable; fix property function roundtripping

[1ec5]

This PR includes several interwoven changes, all related to the runtime styling API:

• MGLStyleLayer properties representing optional style attributes are all null_resettable in Objective-C and therefore implicitly unwrapped optionals in Swift. Instead of returning nil when unset, these properties now return the intrinsic default value. (Fixes MGLStyleLayer properties should be null_resettable or nonnull #6126.)
• Each documentation comment spells out the type of the default value, in anticipation of a fix for Add category convenience methods to NSNumber for style values (was: Runtime styling properties are too loosely typed) #5970.
• When an enumeration-typed property can have either of two values and is set to a constant, its getter returns the right value instead of random garbage.
• When an enumeration-typed property is set to a function, its getter returns that function instead of nil. (Fixes Getters for enumeration-typed style properties return nil instead of functions #6161.)
• Fixed an issue in which setting a padding-typed property to a function would erroneously assert (due to a change omitted from Offsets as vectors, padding as edge insets #6079).
• Fixed an issue in which getting a padding-typed property would yield NSArray-typed stops (another followup to Offsets as vectors, padding as edge insets #6079).
• Added unit tests that caught the four previous issues. (Fixes Test setting style properties to functions #6162.)

/cc @frederoni

[1ec5]

Every enumeration-typed property test is failing on the iOS build bot:

testSymbolLayer, (([(NSValue *)gLayer.iconTextFit isEqualToValue:[MGLRuntimeStylingHelper testEnum:MGLSymbolStyleLayerIconTextFitHeight type:@encode(MGLSymbolStyleLayerIconTextFit)]]) is true) failed

However, I’m unable to reproduce this failure locally.

[1ec5]

The test failures turned out to be legitimate. The tests caught yet another roundtripping issue.

[friedbunny]

Nit: it seems to me that the iOS variant should come before macOS’s.

[1ec5]

Rest assured, I was being completely objective and dispassionate by alphabetizing the items in this list. 😉

[1ec5]

The entire declaration and its comment are now duplicated once per platform using conditional compilation macros. Each comment says only NSColor or UIColor as appropriate. Out of respect for iOS’s primacy, the UIKit version comes first.

[friedbunny]

Other than my iOS partisanship, this LGTM. 👍