sokol_gfx.h: sg_limits should contain a max uniform block size.

[floooh]

E.g. the maximum data size that can be updated with an sg_apply_uniforms() call - this can be down to 16 KByte on mobile GPUs (or even less on very old GLES2 GPUs), so it doesn't make sense to introduce a hardwired limitation.

Then in sg_make_shader(), shader creation should fail if one of the uniform blocks in sg_shader_desc is over the limit, and in addition, the validation function sg_make_shader() should check this too.