mtxOrtho: add option for creating OpenGL style orthographic projections

include/bx/fpumath.h

@@ -715,14 +715,14 @@ namespace bx
 		mtxProjRh(_result, _fovy, _aspect, _near, _far, _oglNdc);
 	}
 
-	inline void mtxOrtho(float* _result, float _left, float _right, float _bottom, float _top, float _near, float _far, float _offset = 0.0f)
+	inline void mtxOrtho(float* _result, float _left, float _right, float _bottom, float _top, float _near, float _far, float _offset = 0.0f, bool _oglNdc = false)
 	{
 		const float aa = 2.0f/(_right - _left);
 		const float bb = 2.0f/(_top - _bottom);
-		const float cc = 1.0f/(_far - _near);
+		const float cc = _oglNdc ? 2.0f/(_far - _near) : 1.0f/(_far - _near);
 		const float dd = (_left + _right)/(_left - _right);
 		const float ee = (_top + _bottom)/(_bottom - _top);
-		const float ff = _near / (_near - _far);
+		const float ff = _oglNdc ? (_near + _far)/(_near - _far) : _near/(_near - _far);
 
 		memset(_result, 0, sizeof(float)*16);
 		_result[ 0] = aa;