enable vsync by default for sdl3 backend, cleanup headers

src/pxl8_io.c

@@ -1,10 +1,13 @@
-#include <ctype.h>
 #include <stdlib.h>
 #include <string.h>
 
 #include "pxl8_io.h"
 #include "pxl8_types.h"
 
+static inline char pxl8_to_lower(char c) {
+    return (c >= 'A' && c <= 'Z') ? c + 32 : c;
+}
+
 pxl8_result pxl8_io_read_file(const char* path, char** content, size_t* size) {
     if (!path || !content || !size) return PXL8_ERROR_NULL_POINTER;
     
@@ -121,7 +124,7 @@ static i32 pxl8_key_code(const char* key_name) {
     char lower_name[64];
     size_t i;
     for (i = 0; i < sizeof(lower_name) - 1 && key_name[i]; i++) {
-        lower_name[i] = (char)tolower((unsigned char)key_name[i]);
+        lower_name[i] = pxl8_to_lower(key_name[i]);
     }
     lower_name[i] = '\0';