FIx merge issues, move preview resolution setting from RNA to BKE, do not clear preview when setting same resolution as current one.

Merge branch 'master' into multi_previews_id

Conflicts:
source/blender/editors/interface/interface_widgets.c

Add copy_from_render to Image RNA API, allows to get 'real' usable image out of render_result one.

Partially revert rBd089bfa9f094.

For now, we do want to keep direct access to raw data as ints/floats.
Time will say whether we want to keep those in final version or not.

Also, minor tweaks to 'set from image' functions.

Ensure given frame index is valid in drawing icon code.

Revert "Somewhat gracefully handle frame index out of bound error."

This is not to be handled in BKE, but in calling code, BKE only asserts
values are valid.

This reverts commit e3fd455d3293f7b8bd15491a3e0193002b648abf.

Merge branch 'master' into multi_previews_id

Don't pass pixels through RNA, but Image pointers.

This makes setting preview images MUCH faster; passing pixels was
O(n^2) in the number of pixels. It also prevents us from having double
setter functions (floats/bytes).

Somewhat gracefully handle frame index out of bound error.

@mont29 We should look at how we handle this properly; I think that the
printf() causes too much console noise.

Replace multi-dim array access to preview image frames by API functions.

Multi-dim array access in RNA is far (very, very far) from being
optimal, so for now let's use functions instead.

Merge branch 'master' into multi_previews_id

Merge branch 'master' into multi_previews_id

Add multi-frame float access in RNA.

Merge branch 'master' into multi_previews_id

Revert "Applied my work from temp-sybren-poselib branch."

This reverts commit ffb0ed305d87d236787163abbe33a921d3eaecf6.

Applied my work from temp-sybren-poselib branch.

Still a bit rough. For example, the viewport used for OpenGL rendering is
more or less randomly chosen from the current screen.

Merge branch 'master' into multi_previews_id

Add preview to Action datablock.

Hopefully did not forget anything.

Some UI access to multi-frames icons...

Not so happy to have to change whole UI API like that, but... can't
really see another solution, since layout API does not return any
handler to button(s) it creates. :/

Multi-previews: basics (not yet usable from end-user code).

This commits adds data and basic API needed to handle multi-previews in
a single PreviewImage struct.

Data model is designed to be fully compatible (backward and forward)
with previous one, while keeping a 'flat array' design to store
previews. It also adds an extra integer per frame, that may be used to
store various meta-data.

This commit also updates write/read code, and adds basic BKE API to handle
multiple frames.

TODO: UI code (only very minimal stuff done so far, probably want to add
some template options too...). Access between icon ID and preview frames
is not yet fully designed, but think will rather go with an extra
integer rather that generating an icon ID/icon struct for every frame.