Massive speed-up. Fixed the format that is set for the renderer. Included runtime checks for XDamage.

--HG--
branch : experimental

1.  Use SDL_X11_HAVE_XRENDER to check for RENDER at runtime.
2.  Added lots of comments.
3.  Added checks and lots of calls to SDL_SetError().
4.  Fixed X11_CreateTexture() so that the pixmap and image created
    are for the format specified by the user and not the window
    format. This is only for the RENDER case.
5.  The above change required that functions to convert SDL
    pixel format enums to Visuals and XRenderPictFormats be added.
6.  Fixed lots of 'style' issues.

Implement blending modes for textures. Fix off-screen pixmaps to be ARGB rather than RGB to enable blending across frames. Clear the screen and off-screen pixmaps when the renderer is created.

Fix so many things that there is little place in this column to list them all but the result is that blending modes just work now for drawing primitives.

Fixes involved:
1. Fix handling of alpha channel when SDL_BLENDMODE_NONE is set.
2. Make xrendercolor use floating-point values for color channels and then convert to 16 bit ints.
3. Fix handling of visuals in SDL_x11modes.c so that a 32 bit ARGB visual is used.
4. Fix the background pixel value in SDL_x11window.c so that the window background has an alpha value of 0xFF and not 0.

Texture rendering mostly works now. Even SDL_TEXTUREACCESS_STREAMING is supported now with a little overhead. Scaling of textures happens using XRender. :D

SDL_Textures should work with XRender now provided that the texture format and screen format match. This is only a temporary limitation.

X11_RenderFillRects and X11_RenderDrawRects use a server side mask pixmap of 1 bit depth now. All drawing on these pixmaps is done by server side functions such as XDrawRectangles and XFillRectangles.

X11_DrawRects now uses a very hacky way of drawing rectangles with XRender. This will be improved in some time.

Fix X11_FillRects to work with double buffering as well as triple buffering. This requires that the buffer pixmap be cleared after every render.

Fix the rendering color channels to be premultiplied with the alpha channel as thats what Xrender expects. Small fixes in X11_CreateTexture. Add some new functions in SDL_x11sym.h as well as support for Xrender.

Xrender uses 16 bit color per channel. Fixed the color handling in X11_RenderFillRects to match this. Xrender just works now for filling rectangles :D .

Modified configure.in to allow building with Xrender. Fixed all problems that prevented compilation.
Builds fine now :)

These include support for drawing a texture using the core protocol
while other textures are drawn using Xrender if Xrender does not support
the color format of the said texture or any other fault with Xrender.

Fixed the XRenderPictureAttributes value in X11_CreateRenderer with graphics_exposures = False.

Start work on Xrender specific additions to X11_TextureData and X11_CreateTexture.

Added some more attributes (specific to Xrender) to the X11 structs.
Added some code for querying the Xrender extension.