--cfg=web_sys_unstable_apis changes position APIs in web_sys to use f64 instead of i32

[Jazzpirate]

if web_sys_unstable_apis is active, several methods in web_sys use f64 instead of i32 (e.g. scroll offsets, mouse positions), which breaks several methods in thaw. To fix this, I added an exported macro to thaw_utils to be as uninvasive as possible, that converts f64 to i32 or vice versa iff web_sys_unstable_apis is active and wrap that around the offending function calls.

To verify that the previous version breaks and this one works, run RUSTFLAGS="--cfg=web_sys_unstable_apis" trunk build in demo.

[Jazzpirate]

Note:

• It seems the change to use f64 happened in a very recent update to web_sys
• The CI fails because of the issue fixed in fix: Recursion limit build error (#445) #446