require explicit engine_name when .html extension is used

[ralphbean]

@amol- reported this in #turbogears

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16:49:33 < amol> threebean: a TG user found an interesting issue with tw2 that raised up only recently, probably after template management change in tw
16:49:45 < amol> it seems that instead of rendering the genshi template of a widget
16:49:53 < amol> it outputs the actual genshi code
16:50:19 < amol> is there anthing that comes to your mind that might cause this issue?
16:50:36 < amol> do I have to change something in the way TG initializes tw2 now?
17:01:27 < amol> Ok, I found the issue :/
17:08:26 < amol> The issue is that if my widget has a genshi template with .html extension and my application (as it uses jinja) has tw2 configured with ['jinja',
'genshi'] preferred templates then tw2 tries to render the template as a Jinja2 template as .html is one of the extensions accepted for Jinja2
17:09:24 < amol> As .html is an extension registed as valid for kajiki, genshi and Jinja2 in tw2.core.middleware.Config by default
17:09:55 < amol> I would suggest requiring to explicitly say the engine_name when .html extension is used, otherwise it would be impossible to render the widget with
the correct engine
17:37:09 < amol> the same issue happens with standard tw2 widgets as far as I see
17:38:00 < amol> as also input_field and so on provide a genshi template which ends with .html extension
17:52:52 < amol> the rendering_extension_lookup option is also totally ignored when looking up for templates, so even changing it doesn't have any effect