Transparency rendered but with terrible ghosting of subpixels when rendering light background with transparent window over a dark color

Add support for glyphs that take more than one cell, spilling into their neighbor. This commit probably adds a performance hit to the CPU side of things which I need to work on reducing in the normal case.

Only allocate PIUs for drawable glyphs. Increase the size of the glyph texture atlas to fit multipart glyphs

Make text transient state have a strong reference to the texture map since it might be removed when the window changes size

Implement a really damn elegant solution to subpixel antialiasing in the GPU renderer, if I don't say so myself.

- Add a frame cursor renderer. Pass screen scale through to all transient states.
- Use a single value for gridsize between frameData and perFrameState
- Subpixel color channels are completely independent. Change models to be keyed on a pair of colors, not a combo of six.  Get bilinear interpolation working! YAY!
- Refactor here and there
- Move inputs to transient state into a new class so it's easier to pass new data to renderers' transient states
- Fork the text fragment shader into a blending and nonblending version
- Draw background image to a temporary texture so the text renderer can sample its background color to do subpixel aa
- Alpha blend default background color over background image

Require a minimum delay between taps of a modifier key for people whose keyboards don't debounce. Issue 6206

Don't draw any lines above the visible range. Their descenders can hang down into the display when vertical spacing is less than 100%. Issue 6223

When there is no terminal window, allow keybindings with the action "select menu item", "new window", "new tab", and "undo" to be executed. Issue 6232

Allow ~ in HTTP URL paths

Fix a crash on launch bug where a selection could have a negative endpoint, causing us to look up a long-gone line. Presumably this occurs because of the 10k line cap on session restoration.

Fix a bug where selection length was not calculated correctly. This affected various optimizations that disable features for long selections. If the column window was not set then a length of up to the grid width (but not greater) was returned.

Allow services that receive text input if up to 10k lines are selected. There has to be a limit because services grab the contents before you know they're needed, at the time the context menu opens. Issue 6218

A single click on the key icon (shown when at a password prompt on localhost) opens the password manager

Rename lastDirectoryIsRemote to lastDirectoryisUnsuitableForOldPWD and add some comments. I always find this code hard to analyze.

Initial text automatically gets a newline sent, so remove the one that used to be added when executing a script.

Issue 6067

Add an advanced pref to use the "correct" algorithm for computing line height, which involves magic incantations. It produces better results for Input Mono Condensed and seems to match what Terminal does. Issue 6159.

Also removed some unneeded baseline offset plumbing.

This allows URLs such as http://sharedserver.com/~homedir/index.html to be
parsed and opened correctly.

Don't make tabs change the background or foreground color of the uninitialized cells they skip over. Issue 6193

Set SUFeedURL in Info.plist. Sometimes Sparkle doesn't see the user defaults key and then dies because it lacks a URL. The down side of this is that beta users may not pull from the beta track and the experiment bucket will get more than its fair share of requests on the stable track. But it beats the heck out of crashing for no reason.

Analysis here:

https://github.com/sparkle-project/Sparkle/issues/1139

Add "check for updates" to menubar icon

Disable tabbing completely. The user defaults hack isn't sufficient because holding down option causes a new window to open as a tab. Issue 6197

Add a beta build configuration, and use it for betas. Change beta, debug, and nightly build configurations to set the preprocessor BETA=1. Add ITBetaAssert, which crashes only in those build configurations. Add some ITBetaAsserts to find why we sometimes try to get negative line numbers.

Prevent calls to retainWindow:withTab with a nil tab, which apparently happens although I don't know how or why. There was a crash log.

Remove some weird logic dating back to commit ee11b549068742112b4d7e3141a9eeece81aa50f in the old-iterm repo that was supposed to "improve function key handling" and made Fn+Ctrl+key send the key ignoring modifiers and Fn+key (without Ctrl) send the key with modifiers, as though Fn weren't pressed. I have no idea what this was meant to do but it caused issue 6176