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/depthstencil.h

@@ -0,0 +1,68 @@
+/**
+ * Copyright (c) 2006-2017 LOVE Development Team
+ *
+ * This software is provided 'as-is', without any express or implied
+ * warranty.  In no event will the authors be held liable for any damages
+ * arising from the use of this software.
+ *
+ * Permission is granted to anyone to use this software for any purpose,
+ * including commercial applications, and to alter it and redistribute it
+ * freely, subject to the following restrictions:
+ *
+ * 1. The origin of this software must not be misrepresented; you must not
+ *    claim that you wrote the original software. If you use this software
+ *    in a product, an acknowledgment in the product documentation would be
+ *    appreciated but is not required.
+ * 2. Altered source versions must be plainly marked as such, and must not be
+ *    misrepresented as being the original software.
+ * 3. This notice may not be removed or altered from any source distribution.
+ **/
+
+#pragma once
+
+namespace love
+{
+namespace graphics
+{
+
+enum StencilAction
+{
+	STENCIL_REPLACE,
+	STENCIL_INCREMENT,
+	STENCIL_DECREMENT,
+	STENCIL_INCREMENT_WRAP,
+	STENCIL_DECREMENT_WRAP,
+	STENCIL_INVERT,
+	STENCIL_MAX_ENUM
+};
+
+enum CompareMode
+{
+	COMPARE_LESS,
+	COMPARE_LEQUAL,
+	COMPARE_EQUAL,
+	COMPARE_GEQUAL,
+	COMPARE_GREATER,
+	COMPARE_NOTEQUAL,
+	COMPARE_ALWAYS,
+	COMPARE_MAX_ENUM
+};
+
+/**
+ * GPU APIs do the comparison in the opposite way of what makes sense for some
+ * of love's APIs. For example in OpenGL if the compare function is GL_GREATER,
+ * then the stencil test will pass if the reference value is greater than the
+ * value in the stencil buffer. With our stencil API it's more intuitive to
+ * assume that setStencilTest(COMPARE_GREATER, 4) will make it pass if the
+ * stencil buffer has a value greater than 4.
+ **/
+CompareMode getReversedCompareMode(CompareMode mode);
+
+bool getConstant(const char *in, StencilAction &out);
+bool getConstant(StencilAction in, const char *&out);
+
+bool getConstant(const char *in, CompareMode &out);
+bool getConstant(CompareMode in, const char *&out);
+
+} // graphics
+} // love