Rename public vars to avoid conflict with library user scope

[offalynne]

GML has some peculiar quirks regarding scope that make library authorship a challenge. Specifically, there are at least three situations in which a variable name is always reserved, and will override instance and local variables when referenced. These are:

1. Asset name
2. Globally scoped function name
3. globalvar name (Deprecated)

For any expression where a library references a variable name as a value, for example type in the expression thisType = type, any of the above 3 cases will result in behaviour undefined by the library, where thisType will be assigned the value of the asset reference, function reference, or globalvar value of type instead of the value of the local or instance variable of the same name as expected.

There is no great solution for this so Input will settle for a compromise -- library variables will be prefixed with two underscores (usually reserved to indicate "private" variables) even when they are exposed by the API as public struct members, as for example with input bindings.

This change is a large effort that touches many corners of the project and will be rolled out gradually.