avcodec/vp3: Optimize alignment check away when possible

Check only on arches that need said check.

(Btw: I do not see how h_loop_filter benefits from alignment
at all and why h_loop_filter_unaligned exists.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/vp3.c

@@ -2031,7 +2031,7 @@ static int vp4_mc_loop_filter(Vp3DecodeContext *s, int plane, int motion_x, int
              plane_height);
 
 #define safe_loop_filter(name, ptr, stride, bounding_values) \
-    if ((uintptr_t)(ptr) & 7) \
+    if (VP3_LOOP_FILTER_NO_UNALIGNED_SUPPORT && (uintptr_t)(ptr) & 7) \
         s->vp3dsp.name##_unaligned(ptr, stride, bounding_values); \
     else \
         s->vp3dsp.name(ptr, stride, bounding_values);

libavcodec/vp3dsp.h

@@ -22,6 +22,10 @@
 #include <stddef.h>
 #include <stdint.h>
 
+// If this is one, {v,h}_loop_filter expect src to be aligned on eight bytes;
+// otherwise they don't have any alignment requirements for src.
+#define VP3_LOOP_FILTER_NO_UNALIGNED_SUPPORT (ARCH_ARM || ARCH_MIPS)
+
 typedef struct VP3DSPContext {
     /**
      * Copy 8xH pixels from source to destination buffer using a bilinear