- Minor fixes in blend operation mechanism.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4783 dfc29bdd-3216-0410-991c-e03cc46cb475
- Prepared OpenGL methods related to separate blending for use.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4784 dfc29bdd-3216-0410-991c-e03cc46cb475
- Minor improvements in D3D8/D3D9 blend operation mechanism.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4785 dfc29bdd-3216-0410-991c-e03cc46cb475