glgsg: Mark SSBO unbound from all binding points upon deletion

Fixes a consistency issue where the buffer might not be rebound properly

panda/src/glstuff/glGraphicsStateGuardian_src.cxx

@@ -7692,6 +7692,12 @@ release_shader_buffer(BufferContext *bc) {
     _current_sbuffer_index = 0;
   }
 
+  for (GLuint &index : _current_sbuffer_base) {
+    if (index == gbc->_index) {
+      index = 0;
+    }
+  }
+
   _glDeleteBuffers(1, &gbc->_index);
   report_my_gl_errors();
 
@@ -7731,6 +7737,12 @@ release_shader_buffers(const pvector<BufferContext *> &contexts) {
       _current_sbuffer_index = 0;
     }
 
+    for (GLuint &index : _current_sbuffer_base) {
+      if (index == gbc->_index) {
+        index = 0;
+      }
+    }
+
     if (debug) {
       GLCAT.debug()
         << "deleting shader buffer " << gbc->_index << "\n";