Store default shader ids

So they can be reused in other shader programs.

include/SDL_gpu.h

···
 	int stored_window_w;
 	int stored_window_h;
 	
+	/*! Shader handles used in the default shader programs */
+	Uint32 default_textured_vertex_shader_id;
+	Uint32 default_textured_fragment_shader_id;
+	Uint32 default_untextured_vertex_shader_id;
+	Uint32 default_untextured_fragment_shader_id;
+	
 	
 	
 	/*! Internal state */

src/renderer_GL_common.inl

···
             return NULL;
         }
 
+        target->context->default_textured_vertex_shader_id = v;
+        target->context->default_textured_fragment_shader_id = f;
         target->context->default_textured_shader_program = p;
 
         // Get locations of the attributes in the shader
···
 
         glUseProgram(p);
 
+        target->context->default_untextured_vertex_shader_id = v;
+        target->context->default_untextured_fragment_shader_id = f;
         target->context->default_untextured_shader_program = target->context->current_shader_program = p;
 
         // Get locations of the attributes in the shader