Add process_command

This allows dynamic control of filter parameters without stopping and 
restarting the filtering process. Wraps avfilter_process_command

---------

Co-authored-by: WyattBlue <wyattblue@auto-editor.com>

Author: caffeinism

av/filter/context.pyi

@@ -12,3 +12,6 @@ class FilterContext:
     def graph(self) -> Graph: ...
     def push(self, frame: Frame) -> None: ...
     def pull(self) -> Frame: ...
+    def process_command(
+        self, cmd: str, arg: str | None = None, res_len: int = 1024, flags: int = 0
+    ) -> str | None: ...

av/filter/context.pyx

@@ -1,3 +1,5 @@
+cimport libav as lib
+
 import weakref
 
 from av.audio.frame cimport alloc_audio_frame
@@ -132,3 +134,41 @@ cdef class FilterContext:
         frame._init_user_attributes()
         frame.time_base = avrational_to_fraction(&self.ptr.inputs[0].time_base)
         return frame
+
+    def process_command(self, cmd, arg=None, int res_len=1024, int flags=0):
+        if not cmd:
+            raise ValueError("Invalid cmd")
+
+        cdef char *c_cmd = NULL
+        cdef char *c_arg = NULL
+
+        c_cmd = cmd
+        if arg is not None:
+            c_arg = arg
+
+        cdef char *c_res = NULL
+        cdef int ret
+        cdef bytearray res_buf = None
+        cdef unsigned char[:] view
+        cdef bytes b
+        cdef int nul
+
+        if res_len > 0:
+            res_buf = bytearray(res_len)
+            view = res_buf
+            c_res = <char*>&view[0]
+        else:
+            c_res = NULL
+
+        with nogil:
+            ret = lib.avfilter_process_command(self.ptr, c_cmd, c_arg, c_res, res_len, flags)
+        err_check(ret)
+
+        if res_buf is not None:
+            b = bytes(res_buf)
+            nul = b.find(b'\x00')
+            if nul >= 0:
+                b = b[:nul]
+            if b:
+                return b.decode("utf-8", "strict")
+        return None

include/libavfilter/avfilter.pxd

@@ -62,6 +62,15 @@ cdef extern from "libavfilter/avfilter.h" nogil:
     # custom
     cdef set pyav_get_available_filters()
 
+    int avfilter_process_command(AVFilterContext *filter,
+                                 const char *cmd,
+                                 const char *arg,
+                                 char *res,
+                                 int res_len,
+                                 int flags)
+
+    cdef int AVFILTER_CMD_FLAG_FAST
+
 
 cdef extern from "libavfilter/buffersink.h" nogil:
     cdef void av_buffersink_set_frame_size(AVFilterContext *ctx, unsigned frame_size)