feat: add slope layer type for DEM-based slope visualization

[eddy-geek]

Implements a new 'slope' layer type that computes slope angle from raster-dem sources using the Horn (1981) algorithm and maps the result to a user-defined color ramp.

Slopes are important for mountain activities like ski-touring.
Example sites: https://www.openslopemap.org/karte/ ; https://snowmap.fr/ ; https://gpx.studio/

All sites today rely on dedicated raster layers which means:

• end-user has no way to customize the colors
• it's inefficient (redundant with other DEM uses like hillshading)

Related style update: maplibre/maplibre-style-spec#1497

Example, using the awesome mapterhorn:

    const map = new maplibregl.Map({
        container: 'map',
        style: {
            version: 8,
            sources: {
                'dem': {
                    type: 'raster-dem',
                    url: 'https://tiles.mapterhorn.com/tilejson.json',
                    tileSize: 256,
                    maxzoom: 16
                },
                'osm': {
                    type: 'raster',
                    tiles: ['https://tile.openstreetmap.org/{z}/{x}/{y}.png'],
                    tileSize: 256
                }
            },
            layers: [
                {
                    id: 'background',
                    type: 'raster',
                    source: 'osm',
                    paint: {
                        'raster-opacity': 1
                    }
                },
                {
                    id: 'slope',
                    type: 'slope',
                    source: 'dem',
                    paint: {
                        'slope-opacity': 0.3,
                        'slope-color': [
                            'interpolate',
                            ['linear'],
                            ['elevation'],
                            0,  '#ffffff',
                            15, '#ffff00',
                            30, '#ff8800',
                            45, '#ff0000',
                            60, '#aa0000',
                            90, '#550000'
                        ]
                    }
                }
            ]
        },
        center: [6.8652, 45.8326],
        zoom: 12
    });

I only needed slope and initially missed #6711 , so I understand we may want to handle this in a more generic way.
Also, multiply blending as per #13583 would improve readability a lot for this use-case too.

New files:

• src/shaders/slope.fragment.glsl — Horn algorithm slope computation with proper meters-per-pixel cell size conversion
• src/shaders/slope.vertex.glsl — vertex shader (based on color-relief)
• src/style/style_layer/slope_style_layer.ts — style layer class with slope color ramp and texture management
• src/render/draw_slope.ts — render function
• src/render/program/slope_program.ts — uniform definitions

Paint properties:

• slope-opacity: number [0-1], default 1
• slope-color: color ramp expression (interpolate over elevation), values represent slope in degrees [0-90]

Registered in: painter.ts, shaders.ts, program_uniforms.ts, create_style_layer.ts

Includes render tests (slope/rainbow, slope/opacity) and a demo HTML.

Note: AI-assisted (Claude), all code reviewed by me

Launch Checklist

• [x ] Confirm your changes do not include backports from Mapbox projects (unless with compliant license) - if you are not sure about this, please ask!
• [x ] Briefly describe the changes in this PR.
• [x ] Link to related issues.
• Include before/after visuals or gifs if this PR includes visual changes.
• [x ] Write tests for all new functionality.
• Document any changes to public APIs.
• Post benchmark scores.
• Add an entry to CHANGELOG.md under the ## main section.