Support for DEPTH16 texture format and its used for PCF shadow maps

[mvaligursky]

Fixes #5355

• adds support for PIXELFORMAT_DEPTH16 pixel format (16bit version of PIXELFORMAT_DEPTH)
• adds new PCF shadow types using this 16bit format: SHADOW_PCF1_FLOAT16, SHADOW_PCF3_FLOAT16, SHADOW_PCF5_FLOAT16 - these work on all light types, both clustered and non-clustered

in a separate PR, existing SHADOW_*** constants will be renamed to be more unified with the new constants.