X 扩展实现

X Server（xorg-server-1.20.8）定义了119个接口（0号和120之外的初始的时候都是ProcBadRequest）。
128之外的接口都是给扩展用的（#define EXTENSION_BASE 128）。
但是每个扩展可能实现了多个接口，而ProcVector仅仅为每个扩展准备了一个入口。所以扩展协议如果有多个接口，就需要自行分发。所以这里的MainProc其实就是扩展协议的入口。对于XRender协议，MainProc就是ProcRenderDispatch。

所以XRenderComposite的分发过程是：
X服务器将来自X客户端的请求通过ProcVector分发给MainProc ProcRenderDispatch。然后ProcRenderDispatch通过ProcRenderVector将命令分发给ProcRenderComposite。
XRrenderComposite => Xlib => ProcRenderDispatch => XRender => ProcRenderComposite
但是XRender本身并没有具体的实现。具体的实现，是通过glamor或者fb实现的。glamor可以通过OpenGL来实现合成。如果没有OpenGL，那么回退到fb。

int (*ProcVector[256]) (ClientPtr /* client */) = {
    ProcBadRequest,
    ProcCreateWindow,
    ProcChangeWindowAttributes,
    ProcGetWindowAttributes,
    ProcDestroyWindow,
    ProcDestroySubwindows,              /* 5 */
    ProcChangeSaveSet,
    ProcReparentWindow,
    ProcMapWindow,
    ProcMapSubwindows,
    ProcUnmapWindow,                    /* 10 */
    ProcUnmapSubwindows,
    ProcConfigureWindow,
    ProcCirculateWindow,
    ProcGetGeometry,
    ProcQueryTree,                      /* 15 */
    ProcInternAtom,
    ProcGetAtomName,
    ProcChangeProperty,
    ProcDeleteProperty,
    ProcGetProperty,                    /* 20 */
    ProcListProperties,
    ProcSetSelectionOwner,
    ProcGetSelectionOwner,
    ProcConvertSelection,
    ProcSendEvent,                      /* 25 */
    ProcGrabPointer,
    ProcUngrabPointer,
    ProcGrabButton,
    ProcUngrabButton,
    ProcChangeActivePointerGrab,        /* 30 */
    ProcGrabKeyboard,
    ProcUngrabKeyboard,
    ProcGrabKey,
    ProcUngrabKey,
    ProcAllowEvents,                    /* 35 */
    ProcGrabServer,
    ProcUngrabServer,
    ProcQueryPointer,
    ProcGetMotionEvents,
    ProcTranslateCoords,                /* 40 */
    ProcWarpPointer,
    ProcSetInputFocus,
    ProcGetInputFocus,
    ProcQueryKeymap,
    ProcOpenFont,                       /* 45 */
    ProcCloseFont,
    ProcQueryFont,
    ProcQueryTextExtents,
    ProcListFonts,
    ProcListFontsWithInfo,              /* 50 */
    ProcSetFontPath,
    ProcGetFontPath,
    ProcCreatePixmap,
    ProcFreePixmap,
    ProcCreateGC,                       /* 55 */
    ProcChangeGC,
    ProcCopyGC,
    ProcSetDashes,
    ProcSetClipRectangles,
    ProcFreeGC,                         /* 60 */
    ProcClearToBackground,
    ProcCopyArea,
    ProcCopyPlane,
    ProcPolyPoint,
    ProcPolyLine,                       /* 65 */
    ProcPolySegment,
    ProcPolyRectangle,
    ProcPolyArc,
    ProcFillPoly,
    ProcPolyFillRectangle,              /* 70 */
    ProcPolyFillArc,
    ProcPutImage,
    ProcGetImage,
    ProcPolyText,
    ProcPolyText,                       /* 75 */
    ProcImageText8,
    ProcImageText16,
    ProcCreateColormap,
    ProcFreeColormap,
    ProcCopyColormapAndFree,            /* 80 */
    ProcInstallColormap,
    ProcUninstallColormap,
    ProcListInstalledColormaps,
    ProcAllocColor,
    ProcAllocNamedColor,                /* 85 */
    ProcAllocColorCells,
    ProcAllocColorPlanes,
    ProcFreeColors,
    ProcStoreColors,
    ProcStoreNamedColor,                /* 90 */
    ProcQueryColors,
    ProcLookupColor,
    ProcCreateCursor,
    ProcCreateGlyphCursor,
    ProcFreeCursor,                     /* 95 */
    ProcRecolorCursor,
    ProcQueryBestSize,
    ProcQueryExtension,
    ProcListExtensions,
    ProcChangeKeyboardMapping,          /* 100 */
    ProcGetKeyboardMapping,
    ProcChangeKeyboardControl,
    ProcGetKeyboardControl,
    ProcBell,
    ProcChangePointerControl,           /* 105 */
    ProcGetPointerControl,
    ProcSetScreenSaver,
    ProcGetScreenSaver,
    ProcChangeHosts,
    ProcListHosts,                      /* 110 */
    ProcChangeAccessControl,
    ProcChangeCloseDownMode,
    ProcKillClient,
    ProcRotateProperties,
    ProcForceScreenSaver,               /* 115 */
    ProcSetPointerMapping,
    ProcGetPointerMapping,
    ProcSetModifierMapping,
    ProcGetModifierMapping,
    ProcBadRequest,                     /* 120 */