Finished it...

src/CRE/shared.cpp

@@ -4,3 +4,13 @@
 */
 
 #include "shared.hpp"
+#include "gl_state.hpp"
+
+void fbo_blit(GLuint src_fbo, GLuint dst_fbo,
+    GLint src_x, GLint src_y, GLint src_width, GLint src_height,
+    GLint dst_x, GLint dst_y, GLint dst_width, GLint dst_height, GLbitfield mask, GLenum filter) {
+    gl_state_bind_read_framebuffer(src_fbo);
+    gl_state_bind_draw_framebuffer(dst_fbo);
+    glBlitFramebuffer(src_x, src_y, src_x + src_width, src_y + src_height,
+        dst_x, dst_y, dst_x + dst_width, dst_y + dst_height, mask, filter);
+}