Accept float values for --max-fps

Android accepts a float value, there is no reason to limit the option
to be an integer.

In particular, it allows to capture at a rate lower than 1 fps. For
example, to capture 1 frame every 5 seconds:

    scrcpy --video-source=camera --max-fps=0.2

It was already possible to pass a float manually:

    scrcpy --video-source=camera \
        --video-codec-options=max-fps-to-encoder:float=0.2

But accepting a float directly for --max-fps is more convenient.

Refs <https://developer.android.com/reference/android/media/MediaFormat#KEY_MAX_FPS_TO_ENCODER>

app/src/options.h

@@ -240,7 +240,7 @@ struct scrcpy_options {
     uint16_t max_size;
     uint32_t video_bit_rate;
     uint32_t audio_bit_rate;
-    uint16_t max_fps;
+    float max_fps;
     enum sc_lock_video_orientation lock_video_orientation;
     enum sc_orientation display_orientation;
     enum sc_orientation record_orientation;