Turn on allocation tracker at run-time and for web

[emilk]

Part of investigating #909

Also:

• Clean up the copied callstack
• Fix copy/paste of web viewer
• Fix backtraces on web

Example web callstack:

wgpu_hal::gles::device::<impl wgpu_hal::Device<wgpu_hal::gles::Api> for wgpu_hal::gles::Device>::create_buffer
wgpu_core::device::queue::prepare_staging_buffer
wgpu_core::device::queue::<impl wgpu_core::hub::Global<G>>::queue_write_buffer
<wgpu::backend::direct::Context as wgpu::context::Context>::queue_write_buffer
<T as wgpu::context::DynContext>::queue_write_buffer
wgpu::Queue::write_buffer
egui_wgpu::renderer::Renderer::update_buffers
<eframe::web::web_painter_wgpu::WebPainterWgpu as eframe::web::web_painter::WebPainter>::paint_and_update_textures
eframe::web::backend::AppRunner::paint
eframe::web::events::paint_and_schedule

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