ARB prog parser: Fix epic memory leak in lexer / parser interface

ARB prog parser: Fix epic memory leak in lexer / parser interface

Anything that matched IDENTIFIER was strdup'ed and returned to the
parser.  However, almost every case of IDENTIFIER in the parser just
dropped the returned string on the floor.  Every swizzle string, every
option string, every use of a variable, etc. leaked memory.

Create a temporary buffer in the parser state (string_dumpster and
dumpster_size).  Return strings from the lexer to the parser in the
buffer.  Grow the buffer as needed.  When the parser needs to keep a
string (i.e., delcaring a new variable), let it make a copy then.

The only leak that valgrind now detects is /occasionally/ the copy of
the program string in gl_program::String is leaked.  I'm not seeing
how. :(