Misc: Fix wrong description & DEBUG_GLX_ERR

- Fix description of "opengl" VSync.

- Add DEBUG_GLX_ERR to check for OpenGL errors.

- Update man page.

src/opengl.h

@@ -12,6 +12,54 @@
 
 #include <ctype.h>
 
+#ifdef DEBUG_GLX_ERR
+
+/**
+ * Get a textual representation of an OpenGL error.
+ */
+static inline const char *
+glx_dump_err_str(GLenum err) {
+  switch (err) {
+    CASESTRRET(GL_NO_ERROR);
+    CASESTRRET(GL_INVALID_ENUM);
+    CASESTRRET(GL_INVALID_VALUE);
+    CASESTRRET(GL_INVALID_OPERATION);
+    CASESTRRET(GL_INVALID_FRAMEBUFFER_OPERATION);
+    CASESTRRET(GL_OUT_OF_MEMORY);
+    CASESTRRET(GL_STACK_UNDERFLOW);
+    CASESTRRET(GL_STACK_OVERFLOW);
+  }
+
+  return NULL;
+}
+
+/**
+ * Check for GLX error.
+ *
+ * http://blog.nobel-joergensen.com/2013/01/29/debugging-opengl-using-glgeterror/
+ */
+static inline void
+glx_check_err_(session_t *ps, const char *func, int line) {
+  if (!ps->glx_context) return;
+
+  GLenum err = GL_NO_ERROR;
+  
+  while (GL_NO_ERROR != (err = glGetError())) {
+    print_timestamp(ps);
+    printf("%s():%d: GLX error ", func, line);
+    const char *errtext = glx_dump_err_str(err);
+    if (errtext) {
+      printf_dbg("%s\n", errtext);
+    }
+    else {
+      printf_dbg("%d\n", err);
+    }
+  }
+}
+
+#define glx_check_err(ps) glx_check_err_(ps, __func__, __LINE__)
+#endif
+
 /**
  * Check if a word is in string.
  */