libavutil: Add av_clip_intp2

there already is a function, av_clip_uintp2() that clips a signed integer
to an unsigned power-of-two range, i.e. 0,2^p-1

this patch adds a function av_clip_intp2() that clips a signed integer
to a signed power-of-two range, i.e. -(2^p),(2^p-1)

the new function can be used as a special case for av_clip(), e.g.
av_clip(x, -8192, 8191) can be rewritten as av_clip_intp2(x, 13)

there are ARM instructions, usat and ssat resp., which map nicely to these
functions (see next patch)
Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavutil/common.h

@@ -152,6 +152,20 @@ static av_always_inline av_const int32_t av_clipl_int32_c(int64_t a)
     else                                         return a;
 }
 
+/**
+ * Clip a signed integer into the -(2^p),(2^p-1) range.
+ * @param  a value to clip
+ * @param  p bit position to clip at
+ * @return clipped value
+ */
+static av_always_inline av_const int av_clip_intp2_c(int a, int p)
+{
+    if ((a + (1 << p)) & ~((1 << (p + 1)) - 1))
+        return (a >> 31) ^ ((1 << p) - 1);
+    else
+        return a;
+}
+
 /**
  * Clip a signed integer to an unsigned power of two range.
  * @param  a value to clip
@@ -386,6 +400,9 @@ static av_always_inline av_const int av_popcount64_c(uint64_t x)
 #ifndef av_clipl_int32
 #   define av_clipl_int32   av_clipl_int32_c
 #endif
+#ifndef av_clip_intp2
+#   define av_clip_intp2    av_clip_intp2_c
+#endif
 #ifndef av_clip_uintp2
 #   define av_clip_uintp2   av_clip_uintp2_c
 #endif