Adds support for Compatility renderer

[pew-jfrohlander]

The assets works overall, but grass displacement does not work properly due to the displacement render texture utilizing the depth texture, and the depth calculations assumes Vulkan depth range which differs from OpenGL.

This PR adds an additional shader global, which is updated in singleton.gd._ready(), setting the value to 0 or 1 based on the renderer used. distance.gdshader uses this value to set the appropriate vec3 to use when calculating depth.

This allows for using the asset with either renderer devices without manually modifying the shader.