metal: set max_buffer_size by the correct physical device restriction

[jinleili]

Connections
The wgpu matrix room mentions the 256MB restriction on macOS several times:

...
wgpu won't stop you, but mac doens't like buffers above 256mb or so
they don't like buffers over 256Mb
Consider it to be 256mb to be safe
...

Description
But starting from iOS 12 and macOS 10.14, there is a maxBufferLength property can used to query max buffer size.

In addition, since [view convertRect: toCoordinateSpace:] only affects x, y coordinates, deleting it from surface fn dimentions does not affect correctness, getting bounds and contentsScale from metalLayer is guaranteed.

Testing
Tested on these devices:

• M1 max MacBook Pro (64G Memory), max_buffer_size = 36864 MB = 36 GB
• Intel MacBook Pro (2018, 32G Memorry), max_buffer_size = 3072 MB = 3 GB
• iPhone 6 Plus, max_buffer_size = 256 MB
• iPad Mini 4, max_buffer_size = 497 MB
• iPhone 8 plus, max_buffer_size = 747 MB
• iPad Pro 2018, max_buffer_size = 947 MB
• iPhone 12 Pro, max_buffer_size = 1433 MB

[kvark]

Looks clean, thank you!