mupdf: fix space and backspace keys

The space key on the penultimate page doesn't work due to an off-by-one.
This is a genuine bug.

The backspace mapping was apparently deliberately removed (just like
'\r', '\n', '=', 'L', 'R' and perhaps others), so I'm less sure we
should fix it. Diff below contains the trivial fix for it as well.

Index: Makefile
===================================================================
RCS file: /var/cvs/ports/textproc/mupdf/Makefile,v
retrieving revision 1.91
diff -u -p -r1.91 Makefile
--- Makefile 3 Aug 2019 19:16:22 -0000 1.91
+++ Makefile 17 Sep 2019 08:43:28 -0000
@@ -5,6 +5,7 @@ COMMENT = graphic library, pdf parser, v
V = 1.16.1
DISTNAME = mupdf-$V-source
PKGNAME = mupdf-${V:S/-rc/rc/}
+REVISION = 0

CATEGORIES = textproc x11

Index: patches/patch-platform_x11_pdfapp_c
===================================================================
RCS file: /var/cvs/ports/textproc/mupdf/patches/patch-platform_x11_pdfapp_c,v
retrieving revision 1.13
diff -u -p -r1.13 patch-platform_x11_pdfapp_c
--- patches/patch-platform_x11_pdfapp_c 3 Aug 2019 19:16:22 -0000 1.13
+++ patches/patch-platform_x11_pdfapp_c 17 Sep 2019 08:31:44 -0000
@@ -28,7 +28,24 @@ Index: platform/x11/pdfapp.c
static int zoom_in(int oldres)
{
int i;
-@@ -1613,9 +1623,9 @@ static void handlescroll(pdfapp_t *app, int modifiers,
+@@ -1471,6 +1481,7 @@ void pdfapp_onkey(pdfapp_t *app, int c, int modifiers)
+ app->pageno++;
+ break;
+
++ case '\b':
+ case 'b':
+ if (app->pany >= 0)
+ {
+@@ -1503,7 +1514,7 @@ void pdfapp_onkey(pdfapp_t *app, int c, int modifiers)
+ {
+ if (app->imgw + app->panx <= app->winw)
+ {
+- if (app->pageno + 1 < app->pagecount)
++ if (app->pageno < app->pagecount)
+ {
+ app->panx = 0;
+ app->pany = 0;
+@@ -1613,9 +1624,9 @@ static void handlescroll(pdfapp_t *app, int modifiers,
{
/* zoom in/out if ctrl is pressed */
if (dir > 0)