Option to have different FPS caps for when the window is active/inactive

[Final23]

I'd love for Scrcpy to have a command line option that limits FPS unrelated to the existing one and is used when the Scrcpy window doesn't have the window focus, i.e. when it runs in the background. The existing option
scrcpy --max-fps=15
should be retained and entirely independent from this new one. I envision something like
scrcpy --max-background-fps=3

I use this in several other apps and games to help me focus by severely reducing frame rate for windows that aren't currently active. It could help with power consumption and network traffic, too, though it may be tricky to have Scrcpy transmit the capture FPS change event to an active session on the fly if we're limiting the capture on the device rather than on the receiving computer.