Minimal FontSize support

[ickshonpe]

Objective

Add support for responsive font sizes supporting rem and viewport units.

Solution

The font_size field of TextFont is now a FontSize.

New FontSize enum:

pub enum FontSize {
    /// Font Size in logical pixels.
    Px(f32),
    /// Font size as a percentage of the viewport width.
    Vw(f32),
    /// Font size as a percentage of the viewport height.
    Vh(f32),
    /// Font size as a percentage of the smaller of the viewport width and height.
    VMin(f32),
    /// Font size as a percentage of the larger of the viewport width and height.
    VMax(f32),
    /// Font Size relative to the value of the `RemSize` resource.
    Rem(f32),
}

FontSize has an eval method that takes viewport and rem values and resolves the actual logical font size.

The text implementations have to pass in the viewport and rem values to TextPipeline's update_buffer and create_text_measure functions, so the font sizes can be evaluated.

Text2d uses the size of the primary window to resolve viewport values (or Vec2::splat(1000) if no primary window found).

There are some limitations because we don't have any sort of font style inheritance yet:

• FontSize values are only reevaluated on changes to TextFont atm, not on changes to the viewport and rem base values.
• Rem units aren't really rem units, and just based on the value of a resource.
• No support for em units.

Testing

The examples should work, but bound to be mistakes as there are so many changes.

The text example has some text that has size in Vh units.

[mnmaita]

Wondering if this PR supersedes #9524?