sdl2: fix CXX_BUILD=1 by hoisting struct sdl2_overlay to file scope

struct sdl2_overlay was defined inline inside the _sdl2_video struct
body in sdl2_common.h. In C the tag leaks to file scope, so
sdl2_gfx.c could freely declare 'struct sdl2_overlay *o'. In C++ a
struct defined inside another struct is scoped to the enclosing type
(_sdl2_video::sdl2_overlay), so under CXX_BUILD=1 the file-scope
references in sdl2_overlay_load() and sdl2_overlays_render() resolved
to a different, incomplete 'sdl2_overlay', producing:

  cannot convert '_sdl2_video::sdl2_overlay*' to
    'sdl2_overlay_load(...)::sdl2_overlay*'
  invalid use of incomplete type 'struct ...::sdl2_overlay'

Move the struct definition out to file scope (still under
HAVE_OVERLAY) and reference it as 'struct sdl2_overlay *overlays;' in
the member. This yields a single shared type in both C and C++; the
layout and the C build are unchanged.

Follow-up to the CXX_BUILD fixes in 059ca26 / 7712d87.

gfx/common/sdl2_common.h

@@ -42,6 +42,22 @@ typedef struct sdl2_tex
    bool rgb32;
 } sdl2_tex_t;
 
+#ifdef HAVE_OVERLAY
+/* On-screen input overlay entry. Defined at file scope (not inside
+ * _sdl2_video) so the tag resolves to the same type in both C and C++;
+ * see the note at the 'overlays' member below. */
+struct sdl2_overlay
+{
+   SDL_Texture *tex;
+   unsigned  tex_w;
+   unsigned  tex_h;
+   float     tex_coords[4];
+   float     vert_coords[4];
+   float     alpha_mod;
+   bool      fullscreen;
+};
+#endif
+
 typedef struct _sdl2_video
 {
    double rotation;
@@ -75,17 +91,16 @@ typedef struct _sdl2_video
     * 0..1 normalised space.  vertex_geom flips y to (1.0f - y) and
     * negates h, matching d3d8 / gl / d3d9_common; vertex_geom
     * comments document the rationale - we undo the flip at render
-    * time in sdl2_overlays_render. */
-   struct sdl2_overlay
-   {
-      SDL_Texture *tex;
-      unsigned  tex_w;
-      unsigned  tex_h;
-      float     tex_coords[4];
-      float     vert_coords[4];
-      float     alpha_mod;
-      bool      fullscreen;
-   } *overlays;
+    * time in sdl2_overlays_render.
+    *
+    * Note: struct sdl2_overlay is defined at file scope (above this
+    * typedef) rather than inline here. Defining a struct inside
+    * another struct's body scopes the tag to the enclosing type in
+    * C++ (sdl2_video::sdl2_overlay), so under CXX_BUILD a file-scope
+    * 'struct sdl2_overlay *' in sdl2_gfx.c would refer to a different,
+    * incomplete type. A top-level definition keeps a single shared
+    * type in both C and C++. */
+   struct sdl2_overlay *overlays;
    unsigned overlays_size;
    bool overlays_enabled;
 #endif