[wip] rlDrawMeshInstanced (#1318)

* rlDrawMeshInstanced first attempt

* rlDrawMeshInstanced OpenGL 3.3 and VAO checks

* rlDrawMeshInstanced GetShaderAttribLocation; comments

* example instanced shader

* RLGL_STANDALONE RAYMATH_STANDALONE Vector4

* apply suggested naming changes; add instanced mesh example

* remove orphan variables

examples/shaders/resources/shaders/glsl330/base_lighting_instanced.vs

@@ -0,0 +1,36 @@
+#version 330
+
+// Input vertex attributes
+in vec3 vertexPosition;
+in vec2 vertexTexCoord;
+in vec3 vertexNormal;
+in vec4 vertexColor;
+
+layout (location = 12) in mat4 instance;
+
+// Input uniform values
+uniform mat4 mvp;
+
+// Output vertex attributes (to fragment shader)
+out vec3 fragPosition;
+out vec2 fragTexCoord;
+out vec4 fragColor;
+out vec3 fragNormal;
+
+// NOTE: Add here your custom variables
+
+void main()
+{
+    // Send vertex attributes to fragment shader
+    fragPosition = vec3(instance * vec4(vertexPosition, 1.0));
+    fragTexCoord = vertexTexCoord;
+    fragColor = vertexColor;
+
+    mat3 normalMatrix = transpose(inverse(mat3(instance)));
+    fragNormal = normalize(normalMatrix * vertexNormal);
+
+    mat4 mvpi = mvp * instance;
+
+    // Calculate final vertex position
+    gl_Position = mvpi * vec4(vertexPosition, 1.0);
+}