add the C library headers MSVC needs

Unqualified uint8_t/size_t and friends were reaching 24 units through
some other header on libstdc++; MSVC's STL does not do that, which broke
gl/ubo.cppm (and made its UBS_PAD macro look like garbage). Also
silences C5244, MSVC's counterpart to the clang warning about glm being
included in the module purview, which is deliberate here.

extras/piped_proc.cppm

@@ -2,6 +2,7 @@
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 module;
+#include <cstddef>
 #ifdef __unix__
 #include <cstdio>
 #endif