Java2D blending - fix swapped SRC and DST
OpenGL blending - fixed SUBTRACT

O = D - (S * SA)
now matches Java2D
OpenGL blending - fixed MULTIPLY

O = D * S
now matches Java2D
OpenGL blending - removed unused functions
OpenGL blending - alpha is added in all modes

OA = SA + DA for all modes
now matches Java2D
Merge pull request #2921 from JakubValtar/bugfix-blendmodes

Multiple blending fixes & improvements