inconsistent behavior of filter pulling from context based on name

Migrated issue, originally created by Michael Bayer (@zzzeek)

this is related to #3 but seems different

from mako.template import Template

x = Template("""
X:
    ${"asdf" | h.foo}
""")

y = Template("""
Y:
    ${"asdf" | z.foo}
""")

z = Template("""
Z:
    ${h}
    ${"asdf" | h.foo}
""")

class h(object):
    foo = str

try:
    print x.render(h=h)
except NameError, e:
    print e

print y.render(z=h)
print z.render(h=h)

Start with the Python reproducer in the issue and compare how the three templates resolve filters from the render context. No source file or test is identified, so locate the filter-resolution entry point and existing tests before deciding what consistent behavior should mean. Done should include a documented expected result and regression coverage for the shown cases.