`_set` suffix was used in two ways (confusing)
* to set a flag to be enabled.
* to set a value passed as an argument.
now use enable/disable rather then set/clear for functions which change flags.
also remove BME_weld.c, the file didnt contain much code and the current extrude works well
* add DM_to_bmesh_ex, DM_to_bmesh for converting a derived mesh to a BMesh (rather than a BMEditMesh)
* have a generic variable for allocsize: bm_mesh_allocsize_default, rather than copying the values about.
* Exploded faces that were meant to be hidden were still created with invalid vertices (0,0,0,0).
* In normal cases this went unnoticed, but for example edge split modifier crashed when it encountered these faces.
Issue was caused by making and loading editMesh in drop_named_image_invoke,
which lead to freeing/changing pointers used by CustomData layers. Some of
this pointers might be used by DerivedMesh, so we need to update DerivedMesh
after loading EditMesh.
These changes are to make the bmesh api more consistent and easier to learn, grouping similar functions which is convenient for autocomplete.
This uses similar convention to RNA.
* use face/loop/edge/vert as a prefix for functions.
* use 'elem' as a prefix too for functions that can take any type with a BMHeader.
* changed from camel case to underscore separated (like RNA).
* Hair particle rotations weren't calculated properly for particle locations along a path and the "particle on path" calculations were not correct in many other ways too.
* Now the particle's location along a path is interpolated directly from the cached paths if it exist. These paths are always correctly calculated.
* Paths are now cached if a particle instance modifier using the particle system with the path option exists.
Problem was in how file selector is called by the effect_strip_add operator invoke. WM_operator_filesel() is not suited, as it directly calls exec if filepath is already set, and sequencer_generic_invoke_xy__internal() will set that path unless otherwise told. So using rather the same way to do as the one used by the other "strip_add" operators...
