Only advertise the SDL_PIXELFORMAT_EXTERNAL_OES format if we can build the shader for it

src/render/opengles2/SDL_shaders_gles2.h

@@ -56,6 +56,7 @@ typedef enum
     GLES2_SHADER_FRAGMENT_TEXTURE_NV21_BT601,
     GLES2_SHADER_FRAGMENT_TEXTURE_NV21_BT709,
 #endif
+    /* Shaders beyond this point are optional and not cached at render creation */
     GLES2_SHADER_FRAGMENT_TEXTURE_EXTERNAL_OES,
     GLES2_SHADER_COUNT
 } GLES2_ShaderType;