mesa/main: Add function to find next higher power of two.

With gcc implementation uses __builtin_clr which counts number of
leading zeros.

Fallback implementation uses bit manipulation. First it duplicates
the highest bit to all lower bits and then adds one to get the power
of two number.

src/mesa/main/imports.h

@@ -405,6 +405,52 @@ _mesa_is_pow_two(int x)
    return !(x & (x - 1));
 }
 
+/**
+ * Round given integer to next higer power of two
+ * If X is zero result is undefined.
+ *
+ * Source for the fallback implementation is
+ * Sean Eron Anderson's webpage "Bit Twiddling Hacks"
+ * http://graphics.stanford.edu/~seander/bithacks.html
+ */
+static INLINE int32_t
+_mesa_next_pow_two_32(uint32_t x)
+{
+#ifdef __GNUC__
+	return 1 << (__builtin_clz(x) ^ 31);
+#else
+	x--;
+	x |= x >> 1;
+	x |= x >> 2;
+	x |= x >> 4;
+	x |= x >> 8;
+	x |= x >> 16;
+	x++;
+	return x;
+#endif
+}
+
+static INLINE int64_t
+_mesa_next_pow_two_64(uint64_t x)
+{
+#ifdef __GNUC__
+	if (sizeof(x) == sizeof(long))
+		return 1 << (__builtin_clzl(x) ^ 63);
+	else
+		return 1 << (__builtin_clzll(x) ^ 63);
+#else
+	x--;
+	x |= x >> 1;
+	x |= x >> 2;
+	x |= x >> 4;
+	x |= x >> 8;
+	x |= x >> 16;
+	x |= x >> 32;
+	x++;
+	return x;
+#endif
+}
+
 
 /***
  *** UNCLAMPED_FLOAT_TO_UBYTE: clamp float to [0,1] and map to ubyte in [0,255]