Fake brightness reimplementation

[hackrfstuff]

Hello. I know this may seem stupid, but a lot of people want this feature back. It was removed for performance issues, but I think that for the time being it does not have a big impact. The people that want it do not seem to mind. This is literally just the pull request that removed the feature, but the other way round.
Also, I thought that this may help individuals who are wanting to implement proper brightness, like through PWM (since they can just clone this).
I have tested it on my device, works like it did originally (no reason why it would not)

[gullradriel]

Indentation needs to be fixed.
And I would prefer that we call get_is_inverted(); and get_dark_cover(); get_brightness(); only once per screen refresh instead of at each pixels.
This will need little modifications, like caching the answer of these calls in the write lock function of the lcd file.
I think we should add static variables for get_is_inverted and get_dark_cover results in firmware/common/lcd_ili9341.hpp, and fill them once in the lcd_start_ram_write function in firmware/common/lcd_ili9341.cpp. or portapack_io.hpp/cpp
Then all calls to get_is_inverted, get_dark_cover, get_brightness should be replaced by their counterpart variable in the class.

[gullradriel]

I'm trying myself at caching the values. We will see.

[gullradriel]

That was good job by @zxkmm .
I only added a bit of caching, and it's running nice.
It's now back in the firmware.