how to filp filament frame buffer upside down into a texture?

[forrestjgq]

Hey guys:

I'm working on rendering filament frame buffer into a texutre_2D so that it could be used for hardware H.264 encoding.
I tried read pixels at each rendering and copy CPU data to texture, it works but not satisfied me for performance reason.

So I changed some filament code so that I can got the pixel buffer from frame buffer, and then copy to texture like this:

    GLRenderTarget const* s = handle_cast<GLRenderTarget const*>(src);
    gl.bindFramebuffer(GL_READ_FRAMEBUFFER, s->gl.fbo);

    GLuint pbo;
    glGenBuffers(1, &pbo);
    gl.bindBuffer(GL_PIXEL_PACK_BUFFER, pbo);
    glBufferData(GL_PIXEL_PACK_BUFFER, p.size, nullptr, GL_STATIC_DRAW);
    glReadPixels(GLint(x), GLint(y), GLint(width), GLint(height), glFormat, glType, nullptr);
    gl.bindBuffer(GL_PIXEL_PACK_BUFFER, 0);

    glBindTexture(GL_TEXTURE_2D, dstName);  // allocated texture 2D
    glBindBuffer(GL_PIXEL_UNPACK_BUFFER, pbo);
    glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, m_width, m_height, GL_RGB, GL_UNSIGNED_BYTE, nullptr);
    glBindTexture(GL_TEXTURE_2D, 0);
    glBindBuffer(GL_PIXEL_UNPACK_BUFFER, 0);

this works, only everything is upside down: the first row becomes the last row...
like this:

I know happens because of NDC, but I don't know how to filp it without coping data to CPU memory.
Is there anyboy could be kind to give me a suggestion?

@yxlao @errissa @ssheorey