[rmodels] Optional GPU skinning (#4321)

* Added optional GPU skinning

* Added skinned bone matrices support for different file formats.

* Moved new shader locations to end of enum to avoid breaking existing examples. Added gpu skinning on drawing of instanced meshes.

* Added GPU skinning example.

* Removed variable declaration to avoid shadowing warning.

examples/models/resources/shaders/glsl100/skinning.fs

@@ -0,0 +1,17 @@
+#version 100
+
+// Input vertex attributes (from vertex shader)
+in vec2 fragTexCoord;
+in vec4 fragColor;
+
+// Output fragment color
+out vec4 finalColor;
+
+uniform sampler2D texture0;
+uniform vec4 colDiffuse;
+
+void main()
+{
+    vec4 texelColor = texture(texture0, fragTexCoord);
+    finalColor = texelColor*colDiffuse*fragColor;
+}