rhi: QRhiGles2 fix leak BlitFromRenderbuffer

Leaked FBO otherwise lead to massive runtime memory consumption increases.

Change-Id: Ic8ffad4917c11294e6c83cdae0b36114d661b251
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/gui/rhi/qrhigles2.cpp

@@ -2501,6 +2501,7 @@ void QRhiGles2::executeCommandBuffer(QRhiCommandBuffer *cb)
                                  GL_COLOR_BUFFER_BIT,
                                  GL_LINEAR);
             f->glBindFramebuffer(GL_FRAMEBUFFER, ctx->defaultFramebufferObject());
+            f->glDeleteFramebuffers(2, fbo);
         }
             break;
         case QGles2CommandBuffer::Command::GenMip: