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/utils/OpenGLBinding.cpp

@@ -74,7 +74,7 @@ namespace utils {
         }
 
         nxtSwapChainError GetNextTexture(nxtSwapChainNextTexture* nextTexture) {
-            nextTexture->texture = reinterpret_cast<void*>(static_cast<size_t>(mBackTexture));
+            nextTexture->texture.u32 = mBackTexture;
             return NXT_SWAP_CHAIN_NO_ERROR;
         }