terminal: Correct logic error in supervisor_start_terminal

If `supervisor_start_terminal` is called twice in a row without
`supervisor_stop_terminal`, it would lose track of a supervisor
allocation.

This can occur when setting the rotation of a display, including the
way that a FramebufferDisplay sets rotation _AFTER_ initial construction,
first with a stack like
```
#0  supervisor_start_terminal
#1  in displayio_display_core_construct
#2  in common_hal_framebufferio_framebufferdisplay_construct
```
and then with a stack like
```
#0  supervisor_start_terminal
#1  in common_hal_framebufferio_framebufferdisplay_construct
#2  in framebufferio_framebufferdisplay_make_new
```
.. without an intervening stop_terminal call.

For reasons I didn't fully explore, this did not become a problem until
the ability to re-allocate a freed supervisor allocation was
implemented in adafruit#3482.  Demonstrating the problem requires adafruit#3482 + adafruit#3497
+ this PR.

Author: jepler

supervisor/shared/display.c

@@ -78,8 +78,20 @@ void supervisor_start_terminal(uint16_t width_px, uint16_t height_px) {
 
     uint16_t total_tiles = width_in_tiles * height_in_tiles;
 
-    // First try to allocate outside the heap. This will fail when the VM is running.
-    tilegrid_tiles = allocate_memory(align32_size(total_tiles), false);
+    // Reuse the previous allocation if possible
+    if (tilegrid_tiles) {
+        if (tilegrid_tiles->length != align32_size(total_tiles)) {
+            free_memory(tilegrid_tiles);
+            tilegrid_tiles = NULL;
+        }
+    }
+    // Next try to allocate outside the heap. This will fail when the VM is running.
+    if (!tilegrid_tiles) {
+        tilegrid_tiles = allocate_memory(align32_size(total_tiles), false);
+    }
+
+    // Finally, allocate inside the heap.  This will be promoted to a GC
+    // allocation at soft reset by supervisor_display_move_memory.
     uint8_t* tiles;
     if (tilegrid_tiles == NULL) {
         tiles = m_malloc(total_tiles, true);