Implement support for 16bit depth buffers shadow maps

[mvaligursky]

Currently, a depth buffer which is part of the render target uses a depth format based on the device:

const depthFormat = device.webgl2 ? gl.DEPTH_COMPONENT32F : gl.DEPTH_COMPONENT16;

We would ideally:

• take an optional parameter when RT is created to specify 16/32bit depth buffer.
• this could be specifically useful for shadow maps to save half the memory bandwidth used
• LightParameters used by the clustered lights would need to specify this, allowing clustered depth texture to be 16bit.

Some parts we'd need to possibly update:

• the resolve depth buffer on webgl2
• grab depth pass functionality would need to reflect the format
• webgl1 implications (could leave untouched perhaps, as shadow depth map is using RGBA8)