Fix compile error against wxWidgets with 2.8 compat

When wxWidgets is compiled in 2.8 compatibility mode, some function
calls become ambiguous.

The fix is pretty simple (except for figuring out where to put the
BITMAPBUNDLE macro) so let me spend the remaining space here by
complaining about how ridiculous the second candidate invocation is.

The intended invocation previously only involved a single implicit
conversion, from wxBitmap to wxBitmapBundle. The unintended one involves
conversions:
- from wxString to wxBitmap
- from wxString to bool
- from wxItemKind (an enum) to wxCoord (typedef'd to int)

src/libresrc/libresrc.h

@@ -18,6 +18,8 @@
 #include "bitmap.h"
 #include "default_config.h"
 
+#include <wx/version.h>
+
 class wxBitmap;
 class wxIcon;
 
@@ -28,3 +30,12 @@ wxIcon libresrc_geticon(const unsigned char *image, size_t size);
 #define GETICON(a) libresrc_geticon(a, sizeof(a))
 
 #define GET_DEFAULT_CONFIG(a) std::string_view(reinterpret_cast<const char *>(a), sizeof(a))
+
+// Compatibility helper: wxWidgets introduced wxBitmapBundle in 3.1.6 and made
+// some functions take wxBitmapBundles rather than wxBitmaps, which makes some
+// function calls ambiguous.
+#if wxCHECK_VERSION(3, 1, 6)
+#define BITMAPBUNDLE(b) wxBitmapBundle(b)
+#else
+#define BITMAPBUNDLE(b) b
+#endif

src/toolbar.cpp

@@ -143,7 +143,7 @@ namespace {
 					wxITEM_NORMAL;
 
 				wxBitmap const& bitmap = command->Icon(icon_size, retina_helper.GetScaleFactor(), GetLayoutDirection());
-				AddTool(TOOL_ID_BASE + commands.size(), command->StrDisplay(context), bitmap, GetTooltip(command), kind);
+				AddTool(TOOL_ID_BASE + commands.size(), command->StrDisplay(context), BITMAPBUNDLE(bitmap), GetTooltip(command), kind);
 
 				commands.push_back(command);
 				needs_onidle = needs_onidle || flags != cmd::COMMAND_NORMAL;

src/visual_tool_vector_clip.cpp

@@ -54,7 +54,7 @@ void VisualToolVectorClip::SetToolbar(wxToolBar *toolBar) {
 
 	int icon_size = OPT_GET("App/Toolbar Icon Size")->GetInt();
 
-#define ICON(name) icon_size == 16 ? GETIMAGE(name ## _16) : GETIMAGE(name ## _24)
+#define ICON(name) BITMAPBUNDLE(icon_size == 16 ? GETIMAGE(name ## _16) : GETIMAGE(name ## _24))
 	toolBar->AddTool(BUTTON_DRAG, _("Drag"), ICON(visual_vector_clip_drag), _("Drag control points"), wxITEM_CHECK);
 	toolBar->AddTool(BUTTON_LINE, _("Line"), ICON(visual_vector_clip_line), _("Appends a line"), wxITEM_CHECK);
 	toolBar->AddTool(BUTTON_BICUBIC, _("Bicubic"), ICON(visual_vector_clip_bicubic), _("Appends a bezier bicubic curve"), wxITEM_CHECK);