Add and update include guards

Include guards in most changed files were missing, I added them keeping
the same style as other SDL files. In some cases I moved the include
guards around to be the first thing the header has to take advantage of
any possible improvements compiler may have for inclusion guards.

src/render/opengl/SDL_shaders_gl.h

@@ -18,6 +18,10 @@
      misrepresented as being the original software.
   3. This notice may not be removed or altered from any source distribution.
 */
+
+#ifndef SDL_shaders_gl_h_
+#define SDL_shaders_gl_h_
+
 #include "../../SDL_internal.h"
 
 /* OpenGL shader implementation */
@@ -44,4 +48,6 @@ extern GL_ShaderContext * GL_CreateShaderContext(void);
 extern void GL_SelectShader(GL_ShaderContext *ctx, GL_Shader shader);
 extern void GL_DestroyShaderContext(GL_ShaderContext *ctx);
 
+#endif /* SDL_shaders_gl_h_ */
+
 /* vi: set ts=4 sw=4 expandtab: */