OpenGL: parallelize tile conversion

During testing of the RAW plugin, I discovered the time to first render (FCP) of a BGRA16U 8k x 5k canvas was abysmally low, ~9 seconds (+ widget overhead). This was because the canvas, although tiled, was converted sequentially and in a single thread.

The simplest speedup, barring SIMD, is to take advantage of the tiles being independent, and parallelize them. This can be done with std::for_each(std::execution::par) (unsupported in Clang with libc++), or with QtConcurrent::blockingMap. We follow the latter here since it's readily available.

This brings the FCP down to less than one second.

Before	After

Test Plan

Build Krita. Check that the canvas's color space conversion still renders correctly.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.