raymath.h: Use C99 inline semantics

RAYMATH_EXTERN_INLINE was renamed to RAYMATH_HEADER_ONLY, which user code
may define if they want to use it as header-only library. If multiple
files in the same project define RAYMATH_HEADER_ONLY, they might each
have duplicate out-of-line definitions of the same functions.

By default, raymath.h exposes inline definitions, which instructs the
compiler _not_ to generate out-of-line definitons, if out-of-line
definitions are required, those of the file defined with
RAYLIB_IMPLEMENTATION are used instead. There may be only one such file.

In C++ mode, the compiler will select only one out-of-line definition
automatically, so no need to define a RAYLIB_IMPLEMENTATION.

Unfortunately, we have to remove raymath function declaration from
raylib.h as those declarations would lead to duplicate out-of-line
definitions which would yield linker errors. This problem didn't
exist with GNU89 or C++, because there multiple defintions are ok,
but in C99 they aren't.

src/raylib.h

@@ -338,14 +338,6 @@ typedef struct Matrix {
     float m3, m7, m11, m15;
 } Matrix;
 
-typedef struct Float3 {
-    float f[3];
-} Float3;
-
-typedef struct Float16 {
-    float f[16];
-} Float16;
-
 // Color type, RGBA (32bit)
 typedef struct Color {
     unsigned char r;
@@ -750,19 +742,6 @@ RLAPI Vector3 ColorToHSV(Color color);                            // Returns HSV
 RLAPI Color GetColor(int hexValue);                               // Returns a Color struct from hexadecimal value
 RLAPI Color Fade(Color color, float alpha);                       // Color fade-in or fade-out, alpha goes from 0.0f to 1.0f
 
-// Math useful functions (available from raymath.h)
-RLAPI Vector3 Vector3Zero(void);                                  // Vector with components value 0.0f
-RLAPI Vector3 Vector3One(void);                                   // Vector with components value 1.0f
-RLAPI Matrix MatrixIdentity(void);                                // Returns identity matrix
-#ifndef Vector3ToFloat
-#define Vector3ToFloat(vec) (Vector3ToFloat_(vec).f)              // Returns Vector3 as float array
-RLAPI Float3 Vector3ToFloat_(Vector3 vec);                        // don't use, use above
-#endif
-#ifndef MatrixToFloat
-#define MatrixToFloat(mat) (MatrixToFloat_(mat).f)                // Returns Matrix as float array
-RLAPI Float16 MatrixToFloat_(Matrix mat);                         // don't use, use above
-#endif
-
 // Misc. functions
 RLAPI void ShowLogo(void);                                        // Activate raylib logo at startup (can be done with flags)
 RLAPI void SetConfigFlags(unsigned char flags);                   // Setup window configuration flags (view FLAGS)