Fixed uninitialized texture id in opengl

This could cause an invalid operation error at texture binding

TODO

@@ -5,7 +5,6 @@ Technlology Preview 2 :
 - Add clouds to OpenGL with 3d textures.
 - Fix potential holes in land rendering (OpenGL and software).
 - Fix polygon culling near the camera in low-res renders (automatic tessellation ?).
-- Fix OpenGL water renderer sometimes not initializing correctly (value error in OpenGL).
 
 Technology Preview 3 :
 - Alter aerial perspective using estimation of the amount of light left after cloud layers traversal.

src/render/opengl/OpenGLVariable.cpp

@@ -17,6 +17,7 @@ OpenGLVariable::OpenGLVariable(const std::string &name):
 {
     type = TYPE_NONE;
     texture_toupload = false;
+    texture_id = 0;
 }
 
 void OpenGLVariable::apply(OpenGLShaderProgram *program, int &texture_unit)