dispatch: Make all API functions non-static.

Some of the functions that we store in the dispatch table are declared
as non-static in their .c files and are inserted into the dispatch
table directly by _mesa_create_exec_table().  Other functions are
declared as static, and are inserted into the dispatch table by a
dedicated function that lives in the same .c file
(e.g. _mesa_loopback_init_api_table() in api_loopback.c).

This patch makes all of these functions non-static, and creates
appropriate prototypes for them, so that in future patches we can
populate the entire dispatch table using a single code-generated
function.
glapi/gen: Comment fix.
glapi/gen: Gather API version info across aliased functions.

gl_XML.py's gl_function class keeps track of an entry_point_api_map
property that tracks, for each set of aliased functions, which ES1 or
ES2 version the given function name first appeared in.

This patch aggregates that information together across aliased
functions, into an easier-to-use api_map property.

Future patches will use this information when code generating
_mesa_create_exec_table(), to determine which set of dispatch table
entries should be populated based on the API.
glapi/gen: handle new XML attributes.

This patch updates gl_XML.py to parse the new XML attributes "exec",
"desktop", "deprecated", and "mesa_name", which will be needed to code
generate _mesa_create_exec_table().
glapi/gen: Add code generation script for _mesa_create_exec_table().

This script generates the file api_exec.c, which contains just the
function _mesa_create_exec_table(), based on the XML files in
src/mapi/glapi/gen.

The following XML attributes, in particular, are used:
- "es1" indicates functions that should be available in ES1 contexts.
- "es2" indicates functions that should be available in ES2/ES3
  contexts.
- "exec" indicates which Mesa function should be dispatched to.  E.g.
  if the GL function is glFoo(), then:
  - exec="mesa" (the default) dispatches to _mesa_Foo().
  - exec="check" dispatches to _check_Foo().
  - exec="es" dispatches to _es_Foo().
  - exec="loopback" dispatches to loopback_Foo().
  - exec="skip" or exec="dynamic" causes this function to be skipped;
    either it is not yet supported ("skip"), or its dispatch table
    entry will be dynamically populated based on GL state ("dynamic").
- "desktop" indicates functions that should be available in desktop GL
  (non-ES) contexts.
- "deprecated" indicates functions that should not be available in
  core contexts.
- "mesa_name" indicates functions whose implementation in Mesa has a
  different suffix than the corresponding GL function name.

The generated code looks roughly like this (showing just a single
statement in each block for brevity):

    struct _glapi_table *
    _mesa_create_exec_table(struct gl_context *ctx)
    {
       struct _glapi_table *exec;

       exec = _mesa_alloc_dispatch_table(_gloffset_COUNT);
       if (exec == NULL)
          return NULL;

       if (_mesa_is_desktop_gl(ctx)) {
          SET_ActiveProgramEXT(exec, _mesa_ActiveProgramEXT);
          /* other functions not shown */
       }
       if (_mesa_is_desktop_gl(ctx) || _mesa_is_gles3(ctx)) {
          SET_BeginQueryARB(exec, _mesa_BeginQueryARB);
          /* other functions not shown */
       }
       if (_mesa_is_desktop_gl(ctx) || ctx->API == API_OPENGLES) {
          SET_GetPointerv(exec, _mesa_GetPointerv);
          /* other functions not shown */
       }
       if (_mesa_is_desktop_gl(ctx) || ctx->API == API_OPENGLES || ctx->API == API_OPENGLES2) {
          SET_ActiveTextureARB(exec, _mesa_ActiveTextureARB);
          /* other functions not shown */
       }
       if (_mesa_is_desktop_gl(ctx) || ctx->API == API_OPENGLES2) {
          SET_AttachShader(exec, _mesa_AttachShader);
          /* other functions not shown */
       }
       if (ctx->API == API_OPENGL) {
          SET_Accum(exec, _mesa_Accum);
          /* other functions not shown */
       }
       if (ctx->API == API_OPENGL || ctx->API == API_OPENGLES) {
          SET_AlphaFunc(exec, _mesa_AlphaFunc);
          /* other functions not shown */
       }
       if (ctx->API == API_OPENGLES) {
          SET_AlphaFuncxOES(exec, _es_AlphaFuncx);
          /* other functions not shown */
       }

       return exec;
    }
dispatch: Code generate api_exec.c.

This patch adjusts makefiles to cause src/mesa/main/api_exec.c to be
generated using src/mapi/glapi/gen/gl_genexec.py.  There should be no
functional change.
dispatch: Delete unused init_dispatch functions.

The new code-generated version of _mesa_create_exec_table() populates
the entire dispatch table (except for dynamic functions) by itself; it
no longer calls separate functions to initialize parts of the dispatch
table.  This patch removes those no-longer-needed functions.