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, 0 deletions

diff --git a/src/image.c b/src/image.c
index 3028c2e707a..d1faadee968 100644
--- a/src/image.c
+++ b/src/image.c
@@ -198,6 +198,9 @@ typedef android_pixmap Pixmap;
 #define GREEN16_FROM_ULONG(color)       (GREEN_FROM_ULONG (color) * 0x101)
 #define BLUE16_FROM_ULONG(color)        (BLUE_FROM_ULONG (color) * 0x101)
 
+/* DPYINFO->n_planes is unsuitable for this file, because it accepts
+   values that may not be supported for pixmap creation.  */
+#define n_planes n_image_planes
 #endif
 
 static void image_disable_image (struct frame *, struct image *);