使用glReadPixels 读取颜色缓存，深度缓存和模板缓存数据

glReadPixels (GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLvoid *pixels);

format和type取值如下

/* PixelCopyType */
#define GL_COLOR                          0x1800
#define GL_DEPTH                          0x1801
#define GL_STENCIL                        0x1802

/* PixelFormat */
#define GL_COLOR_INDEX                    0x1900
#define GL_STENCIL_INDEX                  0x1901
#define GL_DEPTH_COMPONENT                0x1902
#define GL_RED                            0x1903
#define GL_GREEN                          0x1904
#define GL_BLUE                           0x1905
#define GL_ALPHA                          0x1906
#define GL_RGB                            0x1907
#define GL_RGBA                           0x1908
#define GL_LUMINANCE                      0x1909
#define GL_LUMINANCE_ALPHA                0x190A

/* DataType */
#define GL_BYTE                           0x1400
#define GL_UNSIGNED_BYTE                  0x1401
#define GL_SHORT                          0x1402
#define GL_UNSIGNED_SHORT                 0x1403
#define GL_INT                            0x1404
#define GL_UNSIGNED_INT                   0x1405
#define GL_FLOAT                          0x1406
#define GL_2_BYTES                        0x1407
#define GL_3_BYTES                        0x1408
#define GL_4_BYTES                        0x1409
#define GL_DOUBLE                         0x140A