Implement "take a screenshot" feature

* Like in android - press power+volume down, screenshots are saved
  in /sdcard/Pictures/Screenshots (if /sdcard is mounted) or /tmp

Change-Id: Iaefa15b11a1d5fdfac57d77388db1621f378a8d4
Signed-off-by: Vojtech Bocek <vbocek@gmail.com>
diff --git a/minuitwrp/graphics.c b/minuitwrp/graphics.c
index 594592d..e71693a 100644
--- a/minuitwrp/graphics.c
+++ b/minuitwrp/graphics.c
@@ -68,7 +68,7 @@
 static GGLContext *gr_context = 0;
 static GGLSurface gr_font_texture;
 static GGLSurface gr_framebuffer[NUM_BUFFERS];
-static GGLSurface gr_mem_surface;
+GGLSurface gr_mem_surface;
 static unsigned gr_active_fb = 0;
 static unsigned double_buffering = 0;
 static int gr_is_curr_clr_opaque = 0;
@@ -76,7 +76,7 @@
 static int gr_fb_fd = -1;
 static int gr_vt_fd = -1;
 
-static struct fb_var_screeninfo vi;
+struct fb_var_screeninfo vi;
 static struct fb_fix_screeninfo fi;
 
 static bool has_overlay = false;