Dynamically link libraqm

[wiredfool]

We can't currently distribute a binary of Pillow with support for raqm due to license incompatibility, because while libraqm is MIT licensed, it links to GPL libraries. Unfortunately, this means that nearly no one gets support for complex text, as we're providing binaries on the big three platforms now.

This PR replaces the build dependency on libraqm with an attempt to dynamically load the dll, should it be installed by the user. To do this, we're shipping the header from the libraqm project (which is MIT licensed), using this for types, and then dynamically resolving the function addresses at runtime.

To Do:

• fix fedora
• fix osx
• test on windows
• check freebsd
• works on my machine
• release notes,
• instructions

[wiredfool]

Windows: Can't just assign the pointers, need to cast them from FARPROC to a typedef'd version of the function pointer. Annoying but fixable.

_imagingft.c(170) : error C2440: '=' : cannot convert from 'FARPROC' to 'raqm_t *(__cdecl *)(void)'
_imagingft.c(171) : error C2440: '=' : cannot convert from 'FARPROC' to 'int (__cdecl *)(raqm_t *,const uint32_t *,size_t)'
...

Fedora is interesting, it's running but I'm getting corruption from one of the function calls. I'm getting what look like totally reasonable pointers to the functions, but I'm getting some garbage back from raqm_get_glpyhs.

Breakpoint 3, text_layout_raqm (string=0x7f706ceaeb70, self=0x7f7066736f80, dir=0x0, features=0x7f706dff05e0 <_Py_NoneStruct>, 
    glyph_info=0x7ffd534ab140, mask=0) at _imagingft.c:445
445	        (*glyph_info)[i].x_advance = glyphs[i].x_advance;
(gdb) l 
440	    }
441	
442	    for (i = 0; i < count; i++) {
443	        (*glyph_info)[i].index = glyphs[i].index;
444	        (*glyph_info)[i].x_offset = glyphs[i].x_offset;
445	        (*glyph_info)[i].x_advance = glyphs[i].x_advance;
446	        (*glyph_info)[i].y_offset = glyphs[i].y_offset;
447	        (*glyph_info)[i].cluster = glyphs[i].cluster;
448	    }
449	
(gdb) p glyphs[i]
$48 = {index = 570483616, x_advance = -632536784, y_advance = 1766808632, x_offset = 32624, y_offset = 1718968400, cluster = 32624, 
  ftface = 0xfb0ebe7441c223a1}

[wiredfool]

Ok, so Fedora's (old) version had a different definition of the raqm_glyph_t structure, which messed up the function returning an array of them. Windows is compiling now, but it's untested as to if it works or not when raqm is actually installed.

Leaving aside the (current) messy code duplication and lack of having run on Windows, This is now in the state where it's been shown that it's possible to do this. We now need to ask the question: Is this a good idea?

On the plus side, it means that we can distribute support for raqm in the binaries, and people will (hopefully) be able to add libraqm and the dependencies and get text rendering that works for complex languages. On the downside, we're more sensitive to releases of raqm changing the interface on us, and we're opening up a new can of support worms about finding the libraries at runtime. We've not done a dlopen/LoadLibrary before, and I'm guessing that there's going to be at least a few support requests here.

[khaledhosny]

The GPL issue is with FriBiDi, right? Would it be any help if we used a bidi implementation with a non-copyleft license?

[wiredfool]

Maybe, but we've tested this set of libraries. We're not going to do it in this release, so I'll revisit it in a few days. I'm thinking that some sort of versioning symbol would be useful for making sure that we're not interpreting the return value incorrectly.

[wiredfool]

We've now got equivalent support for libraqm on windows as before -- unsupported but may work. I've tested on freebsd, and we're good there.

I think this is a good approach to getting this code distributed in a manner that is license compatible, and should be included in the next release. @homm Comments?