[FFmpeg-devel] Add LoongArch SIMD optimization in swscale lib.

[Hao Chen]

v2: Some modifications were made according to the comments of the reviewers.
v3: Update and run CI test again.
v4: Resolve the warning for the build.

[PATCH v4 1/3] swscale/la: Optimize hscale functions with lasx.
[PATCH v4 2/3] swscale/la: Add yuv2rgb_lasx.c and rgb2rgb_lasx.c
[PATCH v4 3/3] swscale/la: Add output_lasx.c file.