avfilter/af_lv2: call lilv_instance_activate before lilv_instance_run

Why: the change is done to comply with lilv expectations of hosts.

Added call lilv_instance_activate in the config_output function to abide by lilv documentation that states it must be called before lilv_instance_run:
"This MUST be called before calling lilv_instance_run()" - documentation source (https://github.com/lv2/lilv/blob/main/include/lilv/lilv.h)

Added call lilv_instance_deactivate in the uninit function to abide by lv2 documentation:
"If a host calls activate(), it MUST call deactivate() at some point in the future" - documentation source (https://gitlab.com/lv2/lv2/-/blob/main/include/lv2/core/lv2.h)

Added instance_activated integer to LV2Context struct to track if instance was activated and only do lilv_instance_deactivate if was activated to abide by lv2 documentation:
"Hosts MUST NOT call deactivate() unless activate() was previously called." - documentation source (https://gitlab.com/lv2/lv2/-/blob/main/include/lv2/core/lv2.h)

Regarding the patcheck warning (possibly constant :instance_activated):
This is a false positive since the struct member is zero-initialized.

Fixes: trac issue #11661 (https://trac.ffmpeg.org/ticket/11661)
Reported-by: Dave Flater
Signed-off-by: Karl Mogensen <karlmogensen0@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/af_lv2.c

@@ -74,6 +74,7 @@ typedef struct LV2Context {
     float *controls;
 
     LilvInstance *instance;
+    int           instance_activated;
 
     LilvNode  *atom_AtomPort;
     LilvNode  *atom_Sequence;
@@ -387,6 +388,9 @@ static int config_output(AVFilterLink *outlink)
         inlink->min_samples = inlink->max_samples = 4096;
     }
 
+    lilv_instance_activate(s->instance);
+    s->instance_activated = 1;
+
     return 0;
 }
 
@@ -562,6 +566,8 @@ static av_cold void uninit(AVFilterContext *ctx)
 {
     LV2Context *s = ctx->priv;
 
+    if (s->instance_activated)
+        lilv_instance_deactivate(s->instance);
     lilv_node_free(s->powerOf2BlockLength);
     lilv_node_free(s->fixedBlockLength);
     lilv_node_free(s->boundedBlockLength);