Use Timer2 for mouse polling.

This follows a suggestion by Sijmen Schouten in issue #818.
Platoterm64 now works with mouse at 1200 baud.

Bump MOUSE_API_VERSION in asminc/mouse-kernel.inc.

Fix typo in testcode/lib/mouse-test.c.

libsrc/atari/shadow_ram_timerirq2.s

@@ -0,0 +1,81 @@
+;
+; Atari XL shadow RAM timer IRQ #2 handler
+;
+; Christian Groessler, chris@groessler.org, 2019
+;
+
+;DEBUG           =       1
+
+.ifdef __ATARIXL__
+
+SHRAM_HANDLERS  =       1
+                .include        "atari.inc"
+                .include        "romswitch.inc"
+                .export         set_VTIMR2_handler
+
+
+.segment "LOWBSS"
+
+VTIMR2_handler: .res    3
+
+
+.segment "BSS"
+
+old_VTIMR2_handler:
+                .res    2
+
+
+.segment "LOWCODE"
+
+; timer interrupt handler:
+; disable ROM, call user handler, enable ROM again
+
+my_VTIMR2_handler:
+                disable_rom_quick
+                jsr     VTIMR2_handler
+                enable_rom_quick
+                pla
+                rti
+
+.segment "CODE"
+
+; install or remove VTIMR2 handler
+; input: CF - 0/1 for remove/install handler
+;        AX - pointer to handler (if CF=1)
+; registers destroyed
+
+set_VTIMR2_handler:
+
+                bcc     @remove
+
+; install vector
+
+                stx     VTIMR2_handler+2
+                sta     VTIMR2_handler+1        ; save passed vector in low memory
+                lda     #$4C                    ; "JMP" opcode
+                sta     VTIMR2_handler
+
+                lda     VTIMR2
+                sta     old_VTIMR2_handler
+                lda     VTIMR2+1
+                sta     old_VTIMR2_handler+1
+
+                lda     #<my_VTIMR2_handler
+                php
+                sei
+                sta     VTIMR2
+                lda     #>my_VTIMR2_handler
+                sta     VTIMR2+1
+                plp
+                rts
+
+@remove:        php
+                sei
+                lda     old_VTIMR2_handler
+                sta     VTIMR2
+                lda     old_VTIMR2_handler+1
+                sta     VTIMR2+1
+                plp
+                rts
+
+.endif  ; .ifdef __ATARIXL__