glsl: update default precision qualifier when it is set in the shader

glsl: update default precision qualifier when it is set in the shader

Default precision qualifier for a data type could be set several times
inside a shader. This patch allows to update the default precision
qualifier for the given type that is saved in the symbol table.

If it is not in the symbol table, just add it.

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97804
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>