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/backend/opengl/SwapChainGL.cpp

@@ -37,7 +37,7 @@ namespace backend { namespace opengl {
             GetDevice()->HandleError(error);
             return nullptr;
         }
-        GLuint nativeTexture = static_cast<GLuint>(reinterpret_cast<uintptr_t>(next.texture));
+        GLuint nativeTexture = next.texture.u32;
         return new Texture(builder, nativeTexture);
     }