link programs before getting uniform locations; add Widget#draw_rect; fix GL.draw_rect; store uniforms as GLint

src/GL.cc

@@ -46,7 +46,7 @@ void GL_Init()
 {
     ruby_module = rb_define_module("GL");
     rb_define_singleton_method(ruby_module, "draw_rect",
-                               (VALUE(*)(...))GL_draw_rect, 4);
+                               (VALUE(*)(...))GL_draw_rect, 8);
 }
 
 static void load_gl_buffers()

src/GLProgram.h

@@ -17,7 +17,7 @@ enum
 struct GLProgram
 {
     GLuint id;
-    std::unordered_map<int, GLuint> uniforms;
+    std::unordered_map<int, GLint> uniforms;
 };
 
 void GLProgram_Init();