Enable configuring Emacs for "pseudo-grayscale" systems on Android

* doc/emacs/android.texi (Android Windowing): Document how to
configure Emacs for monochrome displays.

* src/androidfns.c (Fx_display_visual_class): Return
Qstatic_gray when n_planes is smaller than 24.
(Fandroid_get_connection): Set n_planes by the value of
android_display_planes.
(syms_of_androidfns): <Qstatic_gray>: New function.

* src/androidterm.c (android_alloc_nearest_color): Allocate
monochrome colors similarly to the X server.
(android_query_colors): Fix typos.
(android_draw_fringe_bitmap): Create bitmaps of n_image_planes
depth.
(android_term_init): Initialize n_image_planes to 24.
(syms_of_androidterm) <android_display_planes>: New variable.

* src/androidterm.h (struct android_display_info): New field
`n_image_planes'.

* src/image.c (n_planes) [HAVE_ANDROID]: Define to
n_image_planes.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/androidterm.h b/src/androidterm.h
index fd4cc99f641..24eb2c30f12 100644
--- a/src/androidterm.h
+++ b/src/androidterm.h
@@ -77,8 +77,9 @@ struct android_display_info
   /* Mouse highlight information.  */
   Mouse_HLInfo mouse_highlight;
 
-  /* Number of planes on this screen.  Always 24.  */
-  int n_planes;
+  /* Number of planes on this screen, and the same for the purposes of
+     image processing.  */
+  int n_planes, n_image_planes;
 
   /* Mask of things causing the mouse to be grabbed.  */
   int grabbed;