Fix T69752: Texture paint sampling colors always 'merged down'

Rgression from rBaf4dcc6073fa.

paint_sample_color > imapaint_pick_face uses the the selection buffer
(DRW_select_buffer_sample_point) and to get flat colors [select_id_flat] we
need to be in SCE_SELECT_FACE mode. This was already fine if you had
'Face Selection Masking' turned on, but got colors including lighting
when turned of [select_id_uniform].

There was already an exception in 'select_cache_init' that turns on
SCE_SELECT_FACE for weightpaint, we just need this for texture paint
(vertex paint) as well... Also moved the logic into
select_id_get_object_select_mode.

Note we were also asserting here:
BLI_assert failed: /blender/source/blender/draw/engines/select/
select_engine.c:174, select_cache_init(), at 'e_data.context.select_mode
!= 0'

Note also this is not working correctly for vertexpaint (yet), but has
been discussed in T69752 and there is a solution by @mano-wii in P1032.

Reviewers: mano-wii

Subscribers: mano-wii

Maniphest Tasks: T69752

Differential Revision: https://developer.blender.org/D5775

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