Align validation with greatest common requirement

While the Metal backend will accept a larger buffer neither
the GL nor the DirectX backend will. By making validation
stricter we prevent code developed with the Metal backend
from crashing when run on GL or DX.

sokol_gfx.h

@@ -15062,7 +15062,7 @@ _SOKOL_PRIVATE bool _sg_validate_apply_uniforms(sg_shader_stage stage_index, int
         SOKOL_VALIDATE(ub_index < stage->num_uniform_blocks, _SG_VALIDATE_AUB_NO_UB_AT_SLOT);
 
         /* check that the provided data size doesn't exceed the uniform block size */
-        SOKOL_VALIDATE(data->size <= stage->uniform_blocks[ub_index].size, _SG_VALIDATE_AUB_SIZE);
+        SOKOL_VALIDATE(data->size == stage->uniform_blocks[ub_index].size, _SG_VALIDATE_AUB_SIZE);
 
         return SOKOL_VALIDATE_END();
     #endif