Fixed some glitch.

src/stereo_camera.hpp

@@ -89,12 +89,12 @@ struct stereo_camera {
         assert(right_raw_image != nullptr);
     }
 
-    bool is_opened() {
+    bool is_opened() const {
         assert(left_camera.is_opened() == right_camera.is_opened());
         return left_camera.is_opened();
     }
 
-    bool is_capturing() {
+    bool is_capturing() const {
         assert(left_camera.is_capturing() == right_camera.is_capturing());
         return left_camera.is_capturing();
     }

src/texture_renderer.cpp

@@ -94,7 +94,7 @@ struct texture_renderer::impl {
         free(info_log);
     }
 
-    void check_program() {
+    void check_program() const {
         GLint status, log_length;
         glGetProgramiv(program, GL_LINK_STATUS, &status);
         glGetProgramiv(program, GL_INFO_LOG_LENGTH, &log_length);