make SWAP macros typesafe using CHECK_TYPE macro.

Its unlikely you want to do short -> int, int -> float etc, conversion during swapping (if its needed we could have a non type checking macro).

Double that the optimized assembler outbut using SWAP() remains unchanged from before.

This exposed quite a few places where redundant type conversion was going on.

Also remove curve.c's swapdata() and replace its use with swap_v3_v3()

source/blender/blenlib/BLI_math_base.h

@@ -137,8 +137,30 @@
 #  endif
 #endif
 
+/* Causes warning:
+ * incompatible types when assigning to type 'Foo' from type 'Bar'
+ * ... the compiler optimizes away the temp var */
+#ifndef CHECK_TYPE
+#ifdef __GNUC__
+#define CHECK_TYPE(var, type)  {  \
+	__typeof(var) *__tmp;         \
+	__tmp = (type *)NULL;         \
+	(void)__tmp;                  \
+} (void)0
+#else
+#define CHECK_TYPE(var, type)
+#endif
+#endif
+
 #ifndef SWAP
-#  define SWAP(type, a, b)  { type sw_ap; sw_ap = (a); (a) = (b); (b) = sw_ap; } (void)0
+#  define SWAP(type, a, b)  {  \
+	type sw_ap;                \
+	CHECK_TYPE(a, type);       \
+	CHECK_TYPE(b, type);       \
+	sw_ap = (a);               \
+	(a) = (b);                 \
+	(b) = sw_ap;               \
+} (void)0
 #endif
 
 #ifndef CLAMP