mesa: Make _mesa_choose_tex_format() choose formats out of a supported table.

Right now this is just tweaking the current code to look at the table.
Choosing actually supported formats will come later.
diff --git a/src/mesa/main/context.c b/src/mesa/main/context.c
index f42a566..2c57c8d 100644
--- a/src/mesa/main/context.c
+++ b/src/mesa/main/context.c
@@ -955,6 +955,14 @@
       ctx->FragmentProgram._MaintainTexEnvProgram = GL_TRUE;
    }
 
+   /* Mesa core handles all the formats that mesa core knows about.
+    * Drivers will want to override this list with just the formats
+    * they can handle, and confirm that appropriate fallbacks exist in
+    * _mesa_choose_tex_format().
+    */
+   memset(&ctx->TextureFormatSupported, GL_TRUE,
+	  sizeof(ctx->TextureFormatSupported));
+
    switch (ctx->API) {
    case API_OPENGL:
 #if FEATURE_dlist