This PR revamps the logic for applying line dash patterns and textures. The prior logic had some holes and inconsistencies, including #741. It implements the following logic in the fragment shader for lines (and also replaces some conditional if/else shader branching with nested mix functions):

• A line can have an active texture sampler in two cases:
  • Implicitly, through a dash pattern (the texture is automatically generated from the dash pattern).
    • The dash pattern defines a foreground color (via the regular color parameter, available in the shader as the vertex color) and background color (via the dash_background_color parameter, available in the fragment shader as u_dash_background_color).
    • The dash texture is a monochrome representation of the pattern, with opaque white for the foreground and transparent black for the background. By using a monochrome texture, the same texture can be reused for the same dash pattern with different foreground/background color combinations.
  • Explicitly, assigned via the texture parameter (in style or draw group).
    • The color values in the texture are meant to be used directly (similar to other style texture sampling behaviors).
• If a line has a dash texture, then in the fragment shader:
  • A uniform u_has_dash indicates this, with a float value of 0 or 1.
  • We determine whether the fragment is in the foreground or background of the dash pattern, and assign the corresponding color, using a mix function controlled by the line texture alpha channel (following the monochrome texture described above).
    • mix(u_dash_background_color, color, _line_color.a), // choose dash foreground or background color
• If the line has an explicit texture, then in the fragment shader:
  • Apply the vertex color to the texture color ("tinting" it), consistent with other similar behaviors, e.g. raster textures on polygons.
    • color * _line_color, // no dash: tint the line texture with the vertex color
• The determination of which texture type (dash pattern or explicit texture) to use is accomplished through another mix function, controlled by the u_has_dash uniform described above.
• Finally, for alpha/blending behavior:
  • When opaque blending is active, a simple "alpha cutout" behavior is applied, in which any alpha pixel with an alpha of less than 0.5 (TANGRAM_ALPHA_TEST) is discarded. This enables basic transparency-like behavior for dashes and textures when using opaque blending.
  • When other blend modes are active, no special logic is applied, and the alpha value resulting from the above color logic will be used as-is.