OpenGL呈现三角形而不是Quad

我当前正在尝试将纹理呈现为全屏QWindow（QT 5.9.1）。
由于纹理都是黑色的，我想确保通过告诉碎片着色器将所有像素呈现为红色，但显然在这里发生了一个错误。

我不是四边形，我在屏幕的左上角得到一个红色的三角形。
我看不到我用代码犯的错误，所以我要求您查看我的代码。
同样，即使我使用glTexImage2D上传的纹理看起来不错，并且glGetError中没有错误。
，也可能仅获得黑色纹理。
预先感谢。

OpenGL上下文和Qwindow的初始化：

int i;
for( i=0;i<QApplication::screens().size();i++ )
{
    if(QApplication::screens().at(i)->name().compare(szScreenName)==0)
        break;
}
if(i==QApplication::screens().size())
    i--;
setGeometry(QApplication::screens().at(i)->availableGeometry());
showFullScreen();
setSurfaceType(QWindow::OpenGLSurface);
QSurfaceFormat fmt;
fmt.setSwapBehavior(QSurfaceFormat::SingleBuffer);
fmt.setRenderableType(QSurfaceFormat::OpenGLES);
setFormat(fmt);
_pContext = new QOpenGLContext(this);
_pContext->setFormat(fmt);
//_pContext->setShareContext(QOpenGLContext::globalShareContext());
_pContext->create();
QObject::connect( this, &DLP::postRenderSignal, this, &DLP::render );
_pContext->makeCurrent(this);
initializeOpenGLFunctions();
//_pTexManager = new TextureManager(QOpenGLContext::globalShareContext()->functions());
_pTexManager = new TextureManager(this);
char* szVertexShader =                          "#version 110n"
                                                "attribute vec4 a_Position;n"
                                                "attribute vec2 a_TexCoord;n"
                                                "varying   vec2 v_TexCoord;n"
                                                "void main()n"
                                                "{n"
                                                "    gl_Position = a_Position;n"
                                                "    v_TexCoord = a_TexCoord;n"
                                                "}n";
char* szFragmentShader =                        "#version 110n"
                                                "precision mediump float;n"
                                                "varying vec2 v_TexCoord;n"
                                                "uniform sampler2D u_Texture;n"
                                                "void main()n"
                                                "{n"
                                                "    gl_FragColor = vec4(1.0,0.0,0.0,1.0);//texture2D( u_Texture, v_TexCoord );n"
                                                "}n";
//_nProgram = linkShader(QOpenGLContext::globalShareContext()->functions(), szVertexShader, szFragmentShader);
_nProgram = linkShader(this, szVertexShader, szFragmentShader);
if(_nProgram!=0)
{
    _nShaderSamplerLoc = glGetUniformLocation(_nProgram,"u_Texture");
    _nShaderTextureLoc = glGetAttribLocation(_nProgram,"a_TexCoord");
    _nShaderPositionLoc = glGetAttribLocation(_nProgram,"a_Position");
}
glGenBuffers(2, _pBuffers );
static const unsigned short pIndexData[] = { 0, 1, 2, 3 };
glBindBuffer(GL_ARRAY_BUFFER, _pBuffers[1]);
glBufferData(GL_ARRAY_BUFFER, sizeof(pIndexData), pIndexData, GL_STATIC_DRAW);
static const float pPosBuffer[20] = {
        -1,   1,   0,    0, 0,
         1,   1,   0,    1, 0,
        -1,  -1,   0,    0, 1,
         1   -1,   0,    1, 1,
};
glBindBuffer(GL_ARRAY_BUFFER, _pBuffers[0]);
glBufferData(GL_ARRAY_BUFFER, 20*4, pPosBuffer, GL_STATIC_DRAW);

渲染代码（DLP :: Render）：

if (!isExposed() || !_pContext)
    return;
_pContext->makeCurrent(this);
glClearColor(0.0, 0.0, 0.0, 1.0);
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT|GL_STENCIL_BUFFER_BIT);
glViewport(0,0,m_nWidth,m_nHeight);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glUseProgram(_nProgram);
glBindBuffer(GL_ARRAY_BUFFER, _pBuffers[0]);
// Load the vertex position
glVertexAttribPointer(_nShaderPositionLoc, 3, GL_FLOAT, false, 5 * 4, (void*)0);
glEnableVertexAttribArray(_nShaderPositionLoc);
// Load the texture coordinate
//glVertexAttribPointer(_nShaderTextureLoc, 2, GL_FLOAT, false, 5 * 4, (void*)(3 * 4));
//glEnableVertexAttribArray(_nShaderTextureLoc);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, _pTexManager->getTexture(key));
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glUniform1i(_nShaderSamplerLoc, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _pBuffers[1]);
glDrawElements(GL_TRIANGLE_STRIP, 4, GL_UNSIGNED_SHORT, 0);
glDisableVertexAttribArray(_nShaderPositionLoc);
//glDisableVertexAttribArray(_nShaderTextureLoc);
glFinish();
_pContext->swapBuffers(this);

编辑：
此外，只有当我在QT中具有多个OpenGL上下文时才发生，例如4 FrameBuffer对象，QML场景图和此Qwindow，因此QT处理OpenGL上下文的问题可能是一个问题。