Support a 16 bits colours mode (RGB565) like the original RPG_RT

[Mimigris]

Describe the issue in detail and how to reproduce it:

Currently EasyRPG only supports a 24 bits mode for colours. While it may be good for new games, this may lead to some discrepancies and unintended things for games made with the 24 bits mode, as reported by a developer of the Yume 2kki Team kuraud in this tweet for example:

Another example that I've found is for the game Ex.Emi, in EasyRPG the ground is red instead of black due to this:
RPG_RT:

EasyRPG:

Having a 16 bits mode for colours should be probably be possible as an option in the EasyRPG.ini as well as a default setting for games that were made using the original RPG_RT.

On another note, we may have issues with transparencies and tones as reported in issue #2202 as well as in the page on the Japanese Production Wiki of Yume 2kki listing differences between RPG_RT and EasyRPG, but this may be related to these differences so this may need more research.