Some changes, additions

[Aransentin]

Hi there! I made some changes to your library:

Functionality:

• Added vec3_reflect() and vec4_reflect().
• Interchanged the order of all loops so that memory is accessed serially, like it was in mat4x4_from_vec3_mul_outer()
• Rewrote mat4x4_invert to one using Laplace expansion instead; the previous one didn't seem to work at all, returning wrong data. The new function also uses 96 multiplications as opposed to the previous one using 160.
• Removed all the memcpy() functions, and instead write directly to the output. You might want to revert this if you had some reason doing that.

Style:

• Fixed spacing everywhere, eg. (v,v) to (v, v)
• Removed braces around single statements, as it was inconsistently applied.
• Added ".f" to all values, so that they are explicitly floats instead of doubles. Also unified style, eg "0.0f" to "0.f"

[datenwolf]

Wow, that's quite some work. However you'll have to add the memcpy stuff again, because by removing it you broke "in-place" operations. Think like

mat4x4 M;
/* do some stuff with M */
mat4x4_mul(M, M, T);
/* so more stuff with M */
mat4x4_transpose(M, M);

are broken now. It's most easy to see for transpose, which is essentially a swap across the diagonal. When writing a swap you must do

temp = a;
a = b;
b = a;

By removing the memcpy, e.g. in transpose you're clobbering the values you're later going to need in that loop.

In invert, Laplace expansion is nice. I did that determinant thing, by having it written out by some code generator (actually I was mostly interested in what a compiler's optimizer would make out of it). However I might replace it with a decomposition based method; using a eigendecomposition based method would give you eigenvalue/eigenvectors code, which again can be used for quaternion from rotation matrix calculations.

[Aransentin]

You are correct about the temporary memory buffers, of course... I re-added them.
I also changed them around a little:

• sizeof() now always takes the source variable (stops gcc -pedantic from complaining, mostly)
• All temporary variables are now named x_ , where x is the name of the variable it takes the place of (some functions where written like that before, so I just implemented it everywhere)

[tdryer]

Looks like @Aransentin's first commit was merged but not the second, so the temporary buffers are currently missing in master and in-place operations are broken.

[Shourn]

just poking at this, because nearly 2 years later in-place operations are still broken :(
would be very nice if @Aransentin second commit could be merged sometime soon

[dizcza]

Do we really need memcpy? We could use vec##n##_copy and mem4x4_dup instead and let the compiler optimize those 2d-loops.
I wonder which would be better on ARM Cortex-M4 in different scenarios: with flags -O0 up to -O3