Show memory report in debug builds

[sqaxomonophonen]

log::debug!() after wgpuSwapChainPresent() which is assumed to be called roughly once per frame. This is useful for detecting memory leaks.

See also: #186

[kvark]

I suggested doing it in debug only, and now I'm having second thoughts. Sorry about this!
This memory dump can be quite verbose if done every frame.
Perhaps, we should just expose it as a C function and let the users call it when needed?

[sqaxomonophonen]

It could be argued that the WebGPU specification itself is lacking a way to get a resource usage report (for the same reason it has .destroy() methods; it implies the possibility of resource management problems even in pure JS+WebGPU applications). So any solution we come with is likely going to be temporary? I'm also fine with adding something like this to wgpu.h:

void wgpuLogResourceUsage(); // calls your WGPULogCallback with WGPULogLevel=0 regardless of log level
// *OR*:
char* wgpuGetResourceUsageString(); // caller "owns" the returned C-string and must free() it

Compared to the implicit logging after wgpuSwapChainPresent() it has a higher risk of breaking forward compatibility, but it's easy to fix (compared with all the other forward compatibility issues). It doesn't add unavoidable verbosity, and it also works better with multi-window programs, or programs that don't want a report every frame.

Anything more complicated than this (like getting the report in a C struct), and I think the effort will we wasted here and better spent discussing it with the WebGPU people. Thankfully, running a custom build of wgpu-native is easy (:+1:), and I'm fine with it for the time being, so feel free to .destroy() this PR if neither solution sits well with you 😃

[kvark]

wgpuGetResourceUsageString sounds good to land here

[sqaxomonophonen]

Does this look good?

I tested with the triangle example, and this works as expected:

diff --git a/examples/triangle/main.c b/examples/triangle/main.c
index 0de6675..4c2a8b0 100644
--- a/examples/triangle/main.c
+++ b/examples/triangle/main.c
@@ -342,6 +342,10 @@ int main() {
     wgpuSwapChainPresent(swapChain);
 
     glfwPollEvents();
+
+    char* usage = wgpuGetResourceUsageString();
+    printf("ru: %s\n", usage);
+    free(usage);
   }
 
   glfwDestroyWindow(window);