vrend: fix a warning with GLES
Fix
[DEBUG:rutabaga_gfx/src/virgl_renderer.rs:104]
GL_IMPLEMENTATION_COLOR_READ_FORMAT is not expected native format 0x80e1 != imp 0x0
with crosvm introduced by commit d9aad06b (vrend: Create frame buffer object in do_readpixels). We can do the warning check only after an fb with read buffer is bound.