Images in labels (text-field)

This PR is an implementation of a proposal in mapbox/mapbox-gl-js#8604.

Style interface

Images could be added to a label via image expression. For example:

"text-field": ["format", "London ", ["image", "london-underground"]]

Implementation details

• Internally, image sections are represented as unicode points from Basic Multilingual Plane Unicode Private Use Area (PUA), so that image positions / style can be preserved after BiDi and/or shaping algorithms are run.
• Quad vertices have a flag indicating whether quad is an sdf image
• Labels that have text and images, are drawn with the new program that has a branch in a fragment shader. Based on an 'sdf' flag value, the branch uses an image atlas to draw a texture or a glyph atlas to draw a text glyph (sdf image)
• SDF images in labels are not supported (could be implemented in follow-up PRs, if needed)

Possible follow-up improvements

• To preserve image size for zoom-dependent text-size values (fractional zoom levels), we could try storing layoutTextSize that was used during layout and pass it as a uniform when label is rendered. Then, image could be scaled according to the size that is interpolated in a shader.
• image-scale option might be added to a formatted image section, so that designers can have more control over the size of an image.

Tests

Expression and render tests are in mapbox/mapbox-gl-js#8904, unit tests are in this PR.