internal: introduce new types to avoid unsigned int rollover and signed int overflow (#7216)

* framebuffer: avoid gluint overflow

GLuint was being initialized to -1 and rolling over to unsigned int max,
its defined behaviour but very unnecessery. add a bool and use it for
checking if allocated or not.

* opengl: avoid gluint rollover

-1 rolls over to unsigned int max, use 0xFF instead.

* core: big uint64_t to int type conversion

there were a few uint64_t to int implicit conversions overflowing int
and causing UB, make all monitor/workspaces/windows use the new
typedefs. also fix the various related 64 to 32 implicit conversions
going around found with -Wshorten-64-to-32

src/signal-safe.cpp

@@ -10,7 +10,7 @@
 extern char** environ;
 
 char const*   sig_getenv(char const* name) {
-    int len = strlen(name);
+    size_t len = strlen(name);
     for (char** var = environ; *var != NULL; var++) {
         if (strncmp(*var, name, len) == 0 && (*var)[len] == '=') {
             return (*var) + len + 1;