Add new variants of love.graphics.clear for controlling how the depth and stencil buffers are cleared.

- Add love.graphics.clear(r, g, b, a [, stencilvalue, depthvalue]).
- Add love.graphics.clear(color1, color2, …, [, stencilvalue, depthvalue]).
- Add love.graphics.clear(false, stencilvalue, depthvalue).

stencilvalue and depthvalue are either true, false, or a number. True is the default behaviour and clears that buffer to the default value (0 for stencil, 1 for depth), false prevents clearing.

The third variant listed above only clears the depth and/or stencil buffers and not the color buffer.

--HG--
branch : minor

src/modules/graphics/Shader.h

@@ -128,6 +128,7 @@ public:
 
 		UniformType baseType;
 		TextureType textureType;
+		bool isDepthTexture;
 		std::string name;
 
 		union