Co-Authored-By: ReinUsesLisp <reinuseslisp@airmail.cc>

i965 (and probably all mesa drivers) require GL_PROGRAM_SEPARABLE when using
glProgramBinary. This is probably required by the standard but it's ignored by
permisive proprietary drivers.

maxwell_3d: Allow texture handles with TIC id zero

gl_rasterizer: Use DSA for textures and move swizzling to texture state

Port citra-emu/citra#4586: "Use QPixmap/QIcon for background color selection button"

Port citra-emu/citra#4583: "citra_qt: Fix saving screenshot when no file extension is provided"

service/nvflinger, service/vi: Improve error case handling

QT: Fix the loading screen 'H' switch logo to not glitch out

Converts many of the Find* functions to return a std::optional<T> as
opposed to returning the raw return values directly. This allows
removing a few assertions and handles error cases like the service
itself does.

video_core/texture: Fix BitField size for depth_minus_one

This member function doesn't actually modify instance state, so it can
be marked as a const member function.

This was missed within #2075. Renames the member function to make it
consistent with the rest of the Find* functions.