Added love.graphics.setColorMask(r, g, b, a) and love.graphics.getColorMask()

setColorMask enables or disables specific color components when rendering and clearing the screen.
For example, setColorMask(true, false, true, true) will prevent the green component of the color of all rendered objects from being written to the current frame buffer.
setColorMask is a wrapper for http://www.opengl.org/sdk/docs/man2/xhtml/glColorMask.xml

--HG--
branch : love.graphics.setColorMask

src/modules/graphics/opengl/wrap_Graphics.h

@@ -70,6 +70,8 @@ int w_setBackgroundColor(lua_State *L);
 int w_getBackgroundColor(lua_State *L);
 int w_setFont(lua_State *L);
 int w_getFont(lua_State *L);
+int w_setColorMask(lua_State *L);
+int w_getColorMask(lua_State *L);
 int w_setBlendMode(lua_State *L);
 int w_setColorMode(lua_State *L);
 int w_setDefaultImageFilter(lua_State *L);