Image rework: part 1 (soundness)

[cyrgani]

Part of #748.

This PR mitigates the unsoundness caused by the Image fields being public and adds new safe methods for immutable access and unsafe methods for mutable access. The second commit changes all usages of these fields within macroquad itself to the new methods.

The reason for this unsoundness is that Image::get_image_data relies on the invariant that width * height == bytes.len() to efficiently convert the image bytes into a color slice.

Fixes #634.
Fixes #746.

New methods

• Image::from_parts: method for creating an image from width, height and bytes while checking if the amount of bytes is correct
• Image::bytes, Image::bytes_mut: returns an immutable / mutable slice of all bytes in the image
• Image::width_mut, Image::height_mut, Image::bytes_vec_mut: unsafe methods that return a mutable reference to the respective field, allowing for unchecked manipulation (mostly exist to support migration of strange usecases)

Other non-breaking changes

• Accessing any of the Image fields directly gives a deprecation warning and suggests using the methods instead.