align commands in the command buffer on 4 bytes

[remjey]

This fixes a crash when microui is used on my 32-bit ARM tablet.
The crash happens when assigning a structure variable directly to a field of an unaligned command union in the command stack.

More specifically, my situation was that I was trying to display a window with a label inside. the window alone would show just fine, but as soon as I added the label inside it, the program crashed.

I managed to narrow down the crash to line 500 of microui.c which happened while adding the label:

cmd->text.pos = pos;

cmd->text.pos was not aligned because the space occupied by the previous text command (for the window title) was not a multiple of 4 because of the length of the text.

With this patch, all commands will occupy a space that has a size which is a multiple of 4, which prevents alignment problems.

[Jan200101]

the command needs to be aligned to the systems alignment (which happens to be 4 in the case of ARM)

size_t alignment = alignof(max_align_t) - 1;
size = (size + alignment) & ~alignment;

is a more portable solution but depends on C11

Since this is an ANSI C library I don't think that is going to get merged in.
an alignment of 8 would probably be better for portability sake, but also more wasteful