nxtSwapChainNextTexture: make texture a union of ptr, u32, u64

Making all textures represented by pointers is a problem for Vulkan
where VkImage is a 64bit type and wouldn't fit in a pointer on 32bit
builds. Make texture contain on of each useful type so that each backend
can choose which one it wants to receive.

src/include/nxt/nxt_wsi.h

@@ -23,7 +23,11 @@ constexpr nxtSwapChainError NXT_SWAP_CHAIN_NO_ERROR = nullptr;
 
 typedef struct {
     /// Backend-specific texture id/name/pointer
-    void* texture = nullptr;
+    union {
+        void* ptr;
+        uint64_t u64;
+        uint32_t u32;
+    } texture;
 } nxtSwapChainNextTexture;
 
 typedef struct {