System files that use a transparent colour (Index 0) on non transparent slots may instead use a black colour while they should not

[Mimigris]

Player platform:

Windows, 64 bits continuous build of the Player.

Attach files (as a .zip archive or link them)

Example system in question:
System.zip

Describe the issue in detail and how to reproduce it:

When using a System file that has a transparent colour used for non transparent graphic slots, like this one for example...

Slots that are expected to not be transparent, like the background colour, the colour of the text or the background of the text will be displayed with a black background in EasyRPG while the RPG_RT will use the colour used in the System file by considering it as not being transparent.
Example with the loading scene:

Could be related to #3012 as well as #2961