Add YGMeasureModeMinContent and support for computing min/max content sizes of nodes

[nicoburns]

Objective

Help make Yoga's flexbox implementation spec compliant.

Context

The spec defines both min-content and max-content sizes, and both are needed to correctly implement CSS layout algorithms. But Yoga currently only supports max-content sizing, which it calls YGMeasureModeUndefined

//    The spec describes four different layout modes: "fill available", "max
//    content", "min content", and "fit content". Of these, we don't use "min
//    content" because we don't support default minimum main sizes (see above
//    for details). Each of our measure modes maps to a layout mode from the
//    spec (https://www.w3.org/TR/CSS3-sizing/#terms):
//      - YGMeasureModeUndefined: max content
//      - YGMeasureModeExactly: fill available
//      - YGMeasureModeAtMost: fit content
//
//    When calling YGNodelayoutImpl and YGLayoutNodeInternal, if the caller
//    passes an available size of undefined then it must also pass a measure
//    mode of YGMeasureModeUndefined in that dimension.

yoga/yoga/Yoga.cpp

Line 2687 in 974473a

// The spec describes four different layout modes: "fill available", "max

Steps

• Rename YGMeasureModeUndefined variant of YGMeasureMode enum to YGMeasureModeMaxContent
• Add YGMeasureModeMinContent variant to YGMeasureMode enum
• Implement min-content sizing logic. This mostly involves passing through min-content to children. But also requires different sizing behaviour when sizing flex-wrap: wrap nodes.