Prevent crashes in Calculator/Eyes DA using ROM FP routines

During calls, MGTK uses the top half of the zero page to hold the
active grafport and local variables. It normally saves/restores the ZP
on each call so that the application's ZP state is preserved. Per
docs, the ZP1 call with param $00 can be used to disable this; MGTK
will write its state to the ZP and assume the ZP is not touched until
a subsequent ZP1 call with param $80, allowing higher performance.

DeskTop would normally run in this state ($00), except when DAs were
invoked since they could use ROM routines (etc) and expect the top
half of the ZP to be preserved during MGTK calls. But it did this with
a toggle ($00, $80, run the DA, then $00 again). Confusingly, the Calc
(and Eyes, c/o copy/pasta) DA would also attempt to get to a normal
state when running ($80, run, $00).

In rev 30532c31 I disabled DeskTop's toggling around DAs to fix an
issue in the Sort Directory DA (see #119), which might have changed
the overall behavior. The result is that on the IIc+ (and possibly
other hardware, but not Virtual II or Laser 128EX) the Calc and Eyes
DAs would crash.

This change disables use of the ZP1 by DeskTop and the DAs. This will
likely impair performance somewhat due to the save/restore overhead on
each call, but at least this gets us to a "known good state".

Fixes #157, #158

desk.acc/calculator.s

@@ -666,7 +666,6 @@ window_title:
 init:   sta     ALTZPON
         lda     LCBANK1
         lda     LCBANK1
-        MGTK_CALL MGTK::SetZP1, preserve_zp_params
         MGTK_CALL MGTK::OpenWindow, winfo
         MGTK_CALL MGTK::InitPort, grafport
         MGTK_CALL MGTK::SetPort, grafport
@@ -696,8 +695,9 @@ loop:   lda     adjust_txtptr_copied-1,x
         bne     loop
 .endproc
 
-        lda     #0              ; Turn off errors
-        sta     ERRFLG
+        lda     #0
+        sta     ERRFLG          ; Turn off errors
+        sta     SHIFT_SIGN_EXT  ; Zero before using FP ops
 
         copy16  #error_hook, COUT_HOOK ; set up FP error handler
 
@@ -772,7 +772,6 @@ ignore_click:
 exit:   MGTK_CALL MGTK::CloseWindow, closewindow_params
         DESKTOP_CALL DT_REDRAW_ICONS
         lda     ROMIN2
-        MGTK_CALL MGTK::SetZP1, overwrite_zp_params
 
 .proc do_close
         ;; Copy following routine to ZP and invoke it

desk.acc/eyes.s

@@ -187,8 +187,6 @@ grow_box_bitmap:
         lda     LCBANK1
         lda     LCBANK1
 
-        ;; Don't let MGTK smash zero page
-        MGTK_CALL MGTK::SetZP1, preserve_zp_params
         lda     #0
         sta     SHIFT_SIGN_EXT  ; Must zero before using FP ops
 
@@ -214,7 +212,6 @@ grow_box_bitmap:
 .proc exit
         MGTK_CALL MGTK::CloseWindow, winfo
         DESKTOP_CALL DT_REDRAW_ICONS
-        MGTK_CALL MGTK::SetZP1, overwrite_zp_params
         rts
 .endproc
 

desktop/desktop_main.s

@@ -1559,10 +1559,7 @@ nope:   dex
         ;; Invoke it
         jsr     set_pointer_cursor
         jsr     reset_grafport3
-        ;;         MGTK_RELAY_CALL MGTK::SetZP1, zp_use_flag0
-        ;;         MGTK_RELAY_CALL MGTK::SetZP1, zp_use_flag1
         jsr     DA_LOAD_ADDRESS
-        ;;         MGTK_RELAY_CALL MGTK::SetZP1, zp_use_flag0
         lda     #0
         sta     running_da_flag
 
@@ -15542,7 +15539,6 @@ end:
         MGTK_RELAY_CALL MGTK::StartDeskTop, startdesktop_params
         jsr     desktop_main::set_mono_mode
         MGTK_RELAY_CALL MGTK::SetMenu, splash_menu
-        MGTK_RELAY_CALL MGTK::SetZP1, zp_use_flag0
         MGTK_RELAY_CALL MGTK::SetCursor, watch_cursor
         MGTK_RELAY_CALL MGTK::ShowCursor
         ;; fall through