* Allow to pass -1 to LLImageBase::reallocateData too (default parameter),
causing it to allocate what is necessary for the size set (same
behavior as allocateData).
* Speed up LLImageRaw::scale with factor 2 or 3 by copying the data less
often and calling LLImageBase::reallocateData instead of destroying
and recreating always.
* Fix gl_rect_2d to not decrement top and right with one pixel.
* Remove the vague "Constrain Proportions" checkbox and replace it with
a new combo box and spinner to explicitly set the target aspect ratio,
allowing to set it also to something different than either the window
aspect ratio (ie, 1.6) or the target image dimension (1.0 for textures)
which was totally lacking when, for example, uploading a profile image
(which needs to be 4:3). This also allows to show snapshots on prims
of arbitrary aspects.
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