Rework symbol visibility for easier maintenance

Save in a few special cases, _X_EXPORT should not be used in C source
files. Instead, it should be used in headers, and the proper C source
include that header. Some special cases are symbols that need to be
shared between modules, but not expected to be used by external drivers,
and symbols that are accessible via LoaderSymbol/dlopen.

  This patch also adds conditionally some new sdk header files, depending
on extensions enabled. These files were added to match pattern for
other extensions/modules, that is, have the headers "deciding" symbol
visibility in the sdk. These headers are:
o Xext/panoramiXsrv.h, Xext/panoramiX.h
o fbpict.h (unconditionally)
o vidmodeproc.h
o mioverlay.h (unconditionally, used only by xaa)
o xfixes.h (unconditionally, symbols required by dri2)

  LoaderSymbol and similar functions now don't have different prototypes,
in loaderProcs.h and xf86Module.h, so that both headers can be included,
without the need of defining IN_LOADER.

  xf86NewInputDevice() device prototype readded to xf86Xinput.h, but
not exported (and with a comment about it).

render/filter.c

@@ -46,7 +46,7 @@ static int  nfilterNames;
  * standard but not required filters don't have constant indices
  */
 
-_X_EXPORT int
+int
 PictureGetFilterId (char *filter, int len, Bool makeit)
 {
     int	    i;
@@ -102,7 +102,7 @@ PictureSetDefaultIds (void)
     return TRUE;
 }
 
-_X_EXPORT char *
+char *
 PictureGetFilterName (int id)
 {
     if (0 <= id && id < nfilterNames)
@@ -123,7 +123,7 @@ PictureFreeFilterIds (void)
     filterNames = 0;
 }
 
-_X_EXPORT int
+int
 PictureAddFilter (ScreenPtr			    pScreen,
 		  char				    *filter,
 		  PictFilterValidateParamsProcPtr   ValidateParams,
@@ -159,7 +159,7 @@ PictureAddFilter (ScreenPtr			    pScreen,
     return id;
 }
 
-_X_EXPORT Bool
+Bool
 PictureSetFilterAlias (ScreenPtr pScreen, char *filter, char *alias)
 {
     PictureScreenPtr    ps = GetPictureScreen(pScreen);
@@ -193,7 +193,7 @@ PictureSetFilterAlias (ScreenPtr pScreen, char *filter, char *alias)
     return TRUE;
 }
 
-_X_EXPORT PictFilterPtr
+PictFilterPtr
 PictureFindFilter (ScreenPtr pScreen, char *name, int len)
 {
     PictureScreenPtr    ps = GetPictureScreen(pScreen);
@@ -244,7 +244,7 @@ convolutionFilterValidateParams (ScreenPtr pScreen,
 }
 
 
-_X_EXPORT Bool
+Bool
 PictureSetDefaultFilters (ScreenPtr pScreen)
 {
     if (!filterNames)
@@ -268,7 +268,7 @@ PictureSetDefaultFilters (ScreenPtr pScreen)
     return TRUE;
 }
 
-_X_EXPORT void
+void
 PictureResetFilters (ScreenPtr pScreen)
 {
     PictureScreenPtr    ps = GetPictureScreen(pScreen);
@@ -278,7 +278,7 @@ PictureResetFilters (ScreenPtr pScreen)
     PictureFreeFilterIds ();
 }
 
-_X_EXPORT int
+int
 SetPictureFilter (PicturePtr pPicture, char *name, int len, xFixed *params, int nparams)
 {
     PictFilterPtr	pFilter;
@@ -312,7 +312,7 @@ SetPictureFilter (PicturePtr pPicture, char *name, int len, xFixed *params, int
     return SetPicturePictFilter (pPicture, pFilter, params, nparams);
 }
 
-_X_EXPORT int
+int
 SetPicturePictFilter (PicturePtr pPicture, PictFilterPtr pFilter,
 		      xFixed *params, int nparams)
 {