Cleanup: reduce amount of math-related includes

Using ClangBuildAnalyzer on the whole Blender build, it was pointing
out that BLI_math.h is the heaviest "header hub" (i.e. non tiny file
that is included a lot).

However, there's very little (actually zero) source files in Blender
that need "all the math" (base, colors, vectors, matrices,
quaternions, intersection, interpolation, statistics, solvers and
time). A common use case is source files needing just vectors, or
just vectors & matrices, or just colors etc. Actually, 181 files
were including the whole math thing without needing it at all.

This change removes BLI_math.h completely, and instead in all the
places that need it, includes BLI_math_vector.h or BLI_math_color.h
and so on.

Change from that:
- BLI_math_color.h was included 1399 times -> now 408 (took 114.0sec
  to parse -> now 36.3sec)
- BLI_simd.h 1403 -> 418 (109.7sec -> 34.9sec).

Full rebuild of Blender (Apple M1, Xcode, RelWithDebInfo) is not
affected much (342sec -> 334sec). Most of benefit would be when
someone's changing BLI_simd.h or BLI_math_color.h or similar files,
that now there's 3x fewer files result in a recompile.

Pull Request #110944

source/blender/python/mathutils/mathutils_Vector.cc

@@ -10,7 +10,9 @@
 
 #include "mathutils.h"
 
-#include "BLI_math.h"
+#include "BLI_math_matrix.h"
+#include "BLI_math_rotation.h"
+#include "BLI_math_vector.h"
 #include "BLI_utildefines.h"
 
 #include "../generic/py_capi_utils.h"