love-blendmode
---------------

Bugs fixed: saveState() and getBlendMode() only supported "alpha" and "additive" modes.
            setBackgroundColor() did not use the alpha parameter.

Cleanups: opengl/Graphics.cpp now makes use of its own get* functions in saveState, instead of making 
          duplicated OpenGL calls and related logic.

New functionality:

There are three major ways to use the blend mode API depending on the amount of flexibility needed. 

1) Simple modes
	
	The old/regular Löve modes.
	Implemented in scripts/graphics.lua.

	setBlendMode(simplemode)

	getBlendMode() returns simplemode
	getBlendModeRGB() returns src, dst, op 		-- the actual settings,
	getBlendModeAlpha() returns src, dst, op	-- same for both

2) Common RGBA src,dst[,op] mode

	Sets the same mode for RGB and A.
	Implemented in scripts/graphics.lua.

	setBlendMode(src, dst)						-- assumed "add"
	setBlendMode(src, dst, op)

	getBlendMode() == src, dst, op				-- or maybe nil here?
	getBlendModeRGB() == src, dst, op
	getBlendModeAlpha() == src, dst, op

3) Separate RGB and A src,dst[,op] modes

	The full OpenGL 1.4 separate blend func/equation.
	Implemented in C++.

	setBlendModeRGB(srcRGB, dstRGB)				-- assumed "add"
	setBlendModeRGB(srcRGB, dstRGB, opRGB)
	setBlendModeAlpha(srcA, dstA)				-- assumed "add"
	setBlendModeAlpha(srcA, dstA, opA)			

	getBlendMode() == nil						-- N/A
	getBlendModeRGB() == srcRGB, dstRGB, opRGB
	getBlendModeAlpha() == srcA, dstA, opA


Implemented:

	simplemode:
		alpha, additive, multiplicative, subtractive

	op operator:
		add, subtract, reverse_subtract, min, max (complete)

	src, dst factors:
		one, zero, src_alpha, one_minus_src_alpha, src_color, one_minus_src_color,
		dst_alpha, one_minus_dst_alpha, dst_color, one_minus_dst_color

		TO-DO: Constant color factors etc. Also, not all factors may be valid for 
			   RGB/A in src/dst etc. This IS defined by the OpenGL spec, but if
			   we want validation in Löve we need to decide on a version to target.
	
Note: I changed the terminology from OpenGL because I think this makes more sense.
      "src,dst factors" = blendfunc parameters
      "op" = blend equation. 
      "mode" = the combination of src,dst,op or simplemode.