gskngltexturelibraryprivate.h: Fix build on Visual Studio

The 'GskNglTextureAtlasEntry' structure is used as a member in two other
structures in two Gsk private headers, namely 'GskNglGlyphValue' and
'GskNglIconData', but the build breaks on Visual Studio since
'GskNglTextureAtlasEntry' contains a last member that is a zero-sized array
that is normally OK if it is the last member of structure -or- if the structure
that contains a 'GskNglTextureAtlasEntry' structure as its last member.
Otherwise, a GCCism is involved here.

Fix the build on Visual Studio compilers by not using a zero-sized array as
a last member of 'GskNglTextureAtlasEntry', on Visual Studio compilers.

gsk/ngl/gskngltexturelibraryprivate.h

@@ -53,6 +53,12 @@ typedef struct _GskNglTextureAtlas
   void *user_data;
 } GskNglTextureAtlas;
 
+#if defined (_MSC_VER) && !defined (__clang__)
+# define NGL_TRAILING_DATA_SIZE 1
+#else
+# define NGL_TRAILING_DATA_SIZE 0
+#endif
+
 typedef struct _GskNglTextureAtlasEntry
 {
   /* A backreference to either the atlas or texture containing
@@ -89,9 +95,11 @@ typedef struct _GskNglTextureAtlasEntry
   /* Suffix data that is per-library specific. gpointer used to
    * guarantee the alignment for the entries using this.
    */
-  gpointer data[0];
+  gpointer data[NGL_TRAILING_DATA_SIZE];
 } GskNglTextureAtlasEntry;
 
+#undef NGL_TRAILING_DATA_SIZE
+
 typedef struct _GskNglTextureLibrary
 {
   GObject        parent_instance;