[rmkit] single-header fixes (#234)

* fix some stbtext extern fields
* fix static framebuffer singleton

---------

Co-authored-by: Brant Martin <brant@brantmartin.com>

Author: rempack-b

src/rmkit/fb/fb.cpy

@@ -1014,7 +1014,7 @@ namespace framebuffer:
       return um
 
 
-  static shared_ptr<FB> _FB
+  extern shared_ptr<FB> _FB = nullptr
 
   // function: framebuffer::get
   // this function returns the app's framebuffer

src/rmkit/fb/stb_text.cpy

@@ -20,8 +20,8 @@
 #define FONT_BUFFER_SIZE 24<<20
 namespace stbtext:
   // TODO: fix the max size read to prevent overflows (or just abort on really large files)
-  static unsigned char font_buffer[FONT_BUFFER_SIZE]
-  static stbtt_fontinfo font;
+  extern unsigned char font_buffer[FONT_BUFFER_SIZE] = {}
+  extern stbtt_fontinfo font = {}
   extern bool did_setup = false
   extern bool GRAYSCALE = false