Misc #204: Add glx_take_screenshot() & others

- Add glx_take_screenshot() for taking a screenshot. With ImageMagick
  the output data could be viewed in this way:

    display -size [SCREEN-WIDTH]x[SCREEN-HEIGHT] -depth 8 -flip
    rgb:[PATH]

  (#204)

- Add D-Bus command `opts_get string:paint_on_overlay_id` to get X
  Composite overlay window ID. (#204)

- Code cleanup.

common.h

@@ -2208,6 +2208,9 @@ glx_render_(session_t *ps, const glx_texture_t *ptex,
 void
 glx_swap_copysubbuffermesa(session_t *ps, XserverRegion reg);
 
+unsigned char *
+glx_take_screenshot(session_t *ps, int *out_length);
+
 #ifdef CONFIG_VSYNC_OPENGL_GLSL
 GLuint
 glx_create_shader(GLenum shader_type, const char *shader_str);
@@ -2484,6 +2487,28 @@ c2_matchd(session_t *ps, win *w, const c2_lptr_t *condlst,
 
 #endif
 
+/**
+ * @brief Dump the given data to a file.
+ */
+static inline bool
+write_binary_data(const char *path, const unsigned char *data, int length) {
+  if (!data)
+    return false;
+  FILE *f = fopen(path, "wb");
+  if (!f) {
+    printf_errf("(\"%s\"): Failed to open file for writing.", path);
+    return false;
+  }
+  int wrote_len = fwrite(data, sizeof(unsigned char), length, f);
+  fclose(f);
+  if (wrote_len != length) {
+    printf_errf("(\"%s\"): Failed to write all blocks: %d / %d", path,
+        wrote_len, length);
+    return false;
+  }
+  return true;
+}
+
 /**
  * @brief Dump raw bytes in HEX format.
  *

compton.c

@@ -1411,6 +1411,20 @@ xr_blur_dst(session_t *ps, Picture tgt_buffer,
   return true;
 }
 
+/*
+ * WORK-IN-PROGRESS!
+static void
+xr_take_screenshot(session_t *ps) {
+  XImage *img = XGetImage(ps->dpy, get_tgt_window(ps), 0, 0,
+      ps->root_width, ps->root_height, AllPlanes, XYPixmap);
+  if (!img) {
+    printf_errf("(): Failed to get XImage.");
+    return NULL;
+  }
+  assert(0 == img->xoffset);
+}
+*/
+
 /**
  * Blur the background of a window.
  */
@@ -7530,6 +7544,16 @@ session_destroy(session_t *ps) {
     ps_g = NULL;
 }
 
+/*
+static inline void
+dump_img(session_t *ps) {
+  int len = 0;
+  unsigned char *d = glx_take_screenshot(ps, &len);
+  write_binary_data("/tmp/dump.raw", d, len);
+  free(d);
+}
+*/
+
 /**
  * Do the actual work.
  *

dbus.c

@@ -901,6 +901,11 @@ cdbus_process_opts_get(session_t *ps, DBusMessage *msg) {
   cdbus_m_opts_get_do(fork_after_register, cdbus_reply_bool);
   cdbus_m_opts_get_do(detect_rounded_corners, cdbus_reply_bool);
   cdbus_m_opts_get_do(paint_on_overlay, cdbus_reply_bool);
+  // paint_on_overlay_id: Get ID of the X composite overlay window
+  if (!strcmp("paint_on_overlay_id", target)) {
+    cdbus_reply_uint32(ps, msg, ps->overlay);
+    return true;
+  }
   cdbus_m_opts_get_do(unredir_if_possible, cdbus_reply_bool);
   cdbus_m_opts_get_do(unredir_if_possible_delay, cdbus_reply_int32);
   cdbus_m_opts_get_do(redirected_force, cdbus_reply_enum);

opengl.c

@@ -1754,6 +1754,32 @@ glx_swap_copysubbuffermesa(session_t *ps, XserverRegion reg) {
   cxfree(rects);
 }
 
+/**
+ * @brief Get tightly packed RGB888 data from GL front buffer.
+ *
+ * Don't expect any sort of decent performance.
+ *
+ * @returns tightly packed RGB888 data of the size of the screen,
+ *          to be freed with `free()`
+ */
+unsigned char *
+glx_take_screenshot(session_t *ps, int *out_length) {
+  int length = 3 * ps->root_width * ps->root_height;
+  GLint unpack_align_old = 0;
+  glGetIntegerv(GL_UNPACK_ALIGNMENT, &unpack_align_old);
+  assert(unpack_align_old > 0);
+  glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
+  unsigned char *buf = cmalloc(length, unsigned char);
+  glReadBuffer(GL_FRONT);
+  glReadPixels(0, 0, ps->root_width, ps->root_height, GL_RGB,
+      GL_UNSIGNED_BYTE, buf);
+  glReadBuffer(GL_BACK);
+  glPixelStorei(GL_UNPACK_ALIGNMENT, unpack_align_old);
+  if (out_length)
+    *out_length = sizeof(unsigned char) * length;
+  return buf;
+}
+
 #ifdef CONFIG_VSYNC_OPENGL_GLSL
 GLuint
 glx_create_shader(GLenum shader_type, const char *shader_str) {