shaders/sampling: refactor pl_shader_sample_ortho

Investingating a bug with broken yuv422 scaling on a cropped texture made me conclude that this API is fundamentally broken. We cannot discard the unused component from inside this function without knowing what the source crop. We need it to be less magic and to move responsibility onto the API user.

Since this is a significant break in API, rename it to pl_shader_sample_ortho_ex and preserve the original name for backwards compatibility. Annoying, but necessary, because otherwise this regresses current users of pl_shader_sample_ortho (e.g. vs-placebo).