All 2D vectors related to image transform code were changed to float2.
Previously, it was decided, that 4x4 matrix should be used for 2D
affine transform, but this is changed to 3x3 now.
Texture painting code did rely on `IMB_transform` with 4x4 matrix.
To avoid large changes, I have added function
`BLI_rctf_transform_calc_m3_pivot_min`.
Main motivation is cleaner code - ease of use of c++ API, and avoiding
returning values by arguments.
Pull Request: https://projects.blender.org/blender/blender/pulls/133692
2a44bdfbd0