[Feature Request]: Implement GS_BGCOLOR support

[fjtrujy]

Description

Performing changes on the GS_BGCOLOR currently doesn't produce any output on the emulation.

The next piece of code:

int main(void) { 
    *((volatile unsigned long int *)0x120000E0) = 0xFFFFFF;
    while(1){}
    return 0; 
}

It turns the screen fully white in the real hardware, however, it does nothing in PCSX2, and the screen remains black.

Reason

To change the GS_BGCOLOR is a common practice done in the PS2DEV to identify in a dummy and quick way if you program is stuck in a concrete step without making the binary way bigger.
This is for instance used in the majority of the elf-loaders.

Cheers.

Examples

PCSX2

Real PS2 Hardware

ELF

Here you have a dummy elf with that specific code

dummy.elf.zip