Created raylib generic uber-shader and custom shaders (markdown)

raylib-generic-uber-shader-and-custom-shaders.md

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+Dealing with custom shaders and make them generic is not an easy task. There are many things to consider for a shader because, after all, the shader is the responsible to process all the data (mesh, materials, textures, lighting) send to the GPU to generate the final frame.
+
+Find an unified generic shader to deal with all kind of stuff is very complicated and, after analysing some of the big engines out there, I decided to go for a custom uber-shader-based solution.
+
+By default, raylib shader struct support the following data:
+```c
+typedef struct Shader {
+    unsigned int id;      // Shader program id
+    
+    // Vertex attributes locations (default locations)
+    int vertexLoc;        // Vertex attribute location point    (default-location = 0)
+    int texcoordLoc;      // Texcoord attribute location point  (default-location = 1)
+    int texcoord2Loc;     // Texcoord2 attribute location point (default-location = 5)
+    int normalLoc;        // Normal attribute location point    (default-location = 2)
+    int tangentLoc;       // Tangent attribute location point   (default-location = 4)
+    int colorLoc;         // Color attibute location point      (default-location = 3)
+
+    // Uniform locations
+    int mvpLoc;           // ModelView-Projection matrix uniform location point (vertex shader)
+    int tintColorLoc;     // Diffuse color uniform location point (fragment shader)
+    
+    // Texture map locations (generic for any kind of map)
+    int mapTexture0Loc;  // Map texture uniform location point (default-texture-unit = 0)
+    int mapTexture1Loc;  // Map texture uniform location point (default-texture-unit = 1)
+    int mapTexture2Loc;  // Map texture uniform location point (default-texture-unit = 2)
+} Shader;
+```
+
+As you can see, most of the location points are pre-defined **on shader loading**; custom shaders developed for raylib must follow those conventions.
+
+On shader loading, following fixed location names are expected for maps:
+```glsl
+uniform sampler2D texture0;   // GL_TEXTURE0
+uniform sampler2D texture1;   // GL_TEXTURE1
+uniform sampler2D texture2;   // GL_TEXTURE2
+```
+
+Shaders are also directly related to Material struct:
+```c
+// Material type
+typedef struct Material {
+    Shader shader;              // Standard shader (supports 3 map textures)
+
+    Texture2D texDiffuse;       // Diffuse texture  (binded to shader mapTexture0Loc)
+    Texture2D texNormal;        // Normal texture   (binded to shader mapTexture1Loc)
+    Texture2D texSpecular;      // Specular texture (binded to shader mapTexture2Loc)
+    
+    Color colDiffuse;           // Diffuse color
+    Color colAmbient;           // Ambient color
+    Color colSpecular;          // Specular color
+    
+    float glossiness;           // Glossiness level (Ranges from 0 to 1000)
+} Material;
+```
+Where three texture maps (texDiffuse, texNormal, texSpecular) are binded to shader location points.
+
+On drawing, depending on textures assigned to material and selected shader, they are internally binded on drawing or not:
+```c
+// Default material loading example
+Material material = LoadDefaultMaterial();                 // Default shader assigned to material (supports  only diffuse map)
+material.texDiffuse = LoadTexture("wood_diffuse.png");     // texture unit 0 activated (available in material shader)
+material.texSpecular = LoadTexture("wood_specular.png");   // texture unit 2 activated (available in material shader)
+
+// Standard material loading example
+Material material = LoadStandardMaterial();                // Standard shader assigned to material (supports diffuse, normal and specular maps)
+material.texDiffuse = LoadTexture("wood_diffuse.png");     // texture unit 0 activated (available in material shader)
+material.texSpecular = LoadTexture("wood_specular.png");   // texture unit 2 NOT activated (not available in material shader)
+```
+
+Despite its name on material struct (`texDiffuse`, `texNormal`, `texSpecular`), user is free to use those maps in any way inside the **custom** shader.