Fixes a stack memory leak and extraneous readDICOMTag XHRs

[floryst]

• Testing minor optimizations to a hot loop in vtkOpenGLTexture by reducing the number of object lookups.
• Preloads the read-dicom-tags pipeline to avoid multiple fetch calls for the same pipeline when reading in a loop. This is less of a concern for image file readers, but it's still good to note it as a potential issue.
• Restores the stack pointer after every main() call, since we do not re-initialize the webworker/emscripten program on every call. Cleans up a stack memory leak. (This makes no assumption on the state of the heap in between main() runs.)

FYI @thewtex @PaulHax the last point above may be a point of discussion for re-using webworkers. Do we continue to treat main() as the entrypoint, or do we instead expose some separate run() function?

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[PaulHax]

Very interesting learning here! Regarding main vs run, I don't know enough about emscripten for an informed opinion.

[thewtex]

@floryst great find and ideas! A few thoughts building on your suggestions on how to move forward here: InsightSoftwareConsortium/ITK-Wasm#760 -- additional thoughts appreciated! Its seems that we could add an option for execution that provides run capabilities. My sense is that this would mesh nicely with usage/ implementation, but we will have to see how the implementation pans out.