Add Depth compare / shadow sampler support. Officially this is only supported in glsl3 shaders.

- Add Texture:setDepthSampleMode(comparemode). Only works on textures with depth pixel formats. A texture with the depth sample mode set will only work with a depth sampler.
- Add DepthImage, DepthArrayImage, and DepthCubeImage sampler keywords to glsl3.

--HG--
branch : minor

src/modules/graphics/wrap_Graphics.lua

@@ -50,6 +50,11 @@ GLSL.SYNTAX = [[
 #define ArrayImage sampler2DArray
 #define CubeImage samplerCube
 #define VolumeImage sampler3D
+#if __VERSION__ >= 300 && !defined(LOVE_GLSL1_ON_GLSL3)
+	#define DepthImage sampler2DShadow
+	#define DepthArrayImage sampler2DArrayShadow
+	#define DepthCubeImage samplerCubeShadow
+#endif
 #define extern uniform
 #ifdef GL_EXT_texture_array
 #extension GL_EXT_texture_array : enable
@@ -78,6 +83,11 @@ GLSL.FUNCTIONS = [[
 	#if __VERSION__ >= 300 || defined(GL_OES_texture_3D)
 		precision lowp sampler3D;
 	#endif
+	#if __VERSION__ >= 300
+		precision lowp sampler2DShadow;
+		precision lowp samplerCubeShadow;
+		precision lowp sampler2DArrayShadow;
+	#endif
 #endif
 
 #if __VERSION__ >= 130 && !defined(LOVE_GLSL1_ON_GLSL3)