Refactor: Add math.h include and DEG2RAD to BLI_math_constants

So it can be used in C code while other BLI code is converted to C++, and so
DNA headers can include it without pulling in full BLI_math_rotation.h.

Pull Request: https://projects.blender.org/blender/blender/pulls/134406

source/blender/blenlib/BLI_math_base.h

@@ -45,17 +45,10 @@
  * - R = result matrix
  */
 
-#if defined(_MSC_VER) && !defined(_USE_MATH_DEFINES)
-#  define _USE_MATH_DEFINES
-#endif
-
 #include "BLI_assert.h"
-#include "BLI_math_inline.h"  // IWYU pragma: export
-#include "BLI_sys_types.h"
-
-#include <math.h>  // IWYU pragma: export
-
 #include "BLI_math_constants.h"  // IWYU pragma: export
+#include "BLI_math_inline.h"     // IWYU pragma: export
+#include "BLI_sys_types.h"
 
 #if defined(__GNUC__)
 #  define NAN_FLT __builtin_nanf("")

source/blender/blenlib/BLI_math_constants.h

@@ -8,6 +8,15 @@
  * \ingroup bli
  */
 
+/* On Windows we have to include math.h before defining our own constants, to
+ * avoid warnings about redefinition. This can likely be simplified later when
+ * code is fully converted to C++, not defining our own constants at all. */
+#if defined(_MSC_VER) && !defined(_USE_MATH_DEFINES)
+#  define _USE_MATH_DEFINES
+#endif
+
+#include <math.h> /* IWYU pragma: export */
+
 #ifndef M_PI
 #  define M_PI 3.14159265358979323846 /* `pi` */
 #endif
@@ -47,3 +56,19 @@
 #ifndef M_LN10
 #  define M_LN10 2.30258509299404568402 /* `log_e 10` */
 #endif
+
+#if defined(_MSC_VER) && !defined(_MATH_DEFINES_DEFINED)
+#  define _MATH_DEFINES_DEFINED
+#endif
+
+/* -------------------------------------------------------------------- */
+/** \name Conversion Defines
+ * \{ */
+
+#define RAD2DEG(_rad) ((_rad) * (180.0 / M_PI))
+#define DEG2RAD(_deg) ((_deg) * (M_PI / 180.0))
+
+#define RAD2DEGF(_rad) ((_rad) * (float)(180.0 / M_PI))
+#define DEG2RADF(_deg) ((_deg) * (float)(M_PI / 180.0))
+
+/** \} */

source/blender/blenlib/BLI_math_rotation.h

@@ -8,7 +8,8 @@
  * \ingroup bli
  */
 
-#include "BLI_math_base.h"  // IWYU pragma: keep
+#include "BLI_math_base.h"       // IWYU pragma: keep
+#include "BLI_math_constants.h"  // IWYU pragma: keep
 #include "BLI_utildefines.h"
 #include "DNA_vec_types.h"