Apply AllocationLimitMegabytes against the total size of multi-frame images before attempting to decode them

[antonfirsov]

Configuration.Default.MemoryAllocator = MemoryAllocator.Create(new MemoryAllocatorOptions()
{
    AllocationLimitMegabytes = 16
});

// Should throw InvalidMemoryOperationException when the total size of all frames to allocate for this image are over 16MB, even if the individual frame sizes are below 16MB.
var img = Image.Load("multiframe.gif");

Even though strictly speaking this would be a hack since the allocation limit is originally meant to be per-buffer, I think this is something users want us to do. For built-in decoders, this is possible to implement by adding an internal validation method toMemoryAllocator; we can figure out later whether we want to make the feature available to external decoders.

[antonfirsov]

Posted as a discussion since the new repo setup doesn't allow godmode issue creation 😅 Converted to #2850 now.