Fix creation of RGB texture with QOpenGLTexture/Qt3D on OpenGL ES 2.0

Modification of the function pixelFormatCompatibleWithInternalFormat to
not change RGB pixel format to RGBA one, allowing the creation of valid
RGB textures with Qt3D.

Task-number: QTBUG-66365
Change-Id: I5c3187a3fefaedf85140f80fbb7145e1a762805b
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/gui/opengl/qopengltexture.cpp

@@ -778,6 +778,8 @@ static QOpenGLTexture::PixelFormat pixelFormatCompatibleWithInternalFormat(QOpen
         return QOpenGLTexture::Alpha;
 
     case QOpenGLTexture::RGBFormat:
+        return QOpenGLTexture::RGB;
+
     case QOpenGLTexture::RGBAFormat:
         return QOpenGLTexture::RGBA;