Remove mipmapping + anisotropic filtering for raster tiles

[kkaefer]

We added a call to generateMipMap on a hunch in #112 a few years ago, and it has remained in the raster drawing code ever since. Mipmaps only make sense when the texture is scaled down significantly (way below 50% of its original size), as a mean to improve the quality of the texels used to calculate the fragment color. A few weeks ago, we added anisotropic filtering to improve the quality.

We never added mipmapping for raster tiles in Native, and I don't think we need to, given that we almost always draw the textures in a size that isn't smaller than half the original dimensions. Since we don't mipmap raster tiles, we also don't need anisotropic filtering.

We should remove the call to generateMipMap and consequently remove anisotropic filtering for raster tiles. This should achieve the near identical visual quality while reducing memory usage and making the code less complex.

mapbox/mapbox-gl-native#8688

[andrewharvey]

Do you propose to use gl.LINEAR/gl.NEAREST? Based on that the current behaviour looks smoother than whan I'm seeing with gl.LINEAR without Anisotropic Filtering.

TEXTURE_MIN_FILTER = gl.LINEAR_MIPMAP_NEAREST with Anisotropic Filtering (master)

TEXTURE_MIN_FILTER = gl.LINEAR without mipmaps without Anisotropic Filtering

TEXTURE_MIN_FILTER = gl.LINEAR_MIPMAP_NEAREST without Anisotropic Filtering

TEXTURE_MIN_FILTER = gl.NEAREST without mipmaps without Anisotropic Filtering

[kkaefer]

Ah interesting; I was developing this from a GL native side, and didn't add mipmapping, which means anisotropic filtering has no effect, so I concluded that it doesn't have an effect. When comparing the images, you'll notice that anisotropic filtering + mipmapping only has an effect when the image is scaled down by more than 50% (in the upper half of the image). The ideal solution would be to make the mipmap generation conditional, and only start generating mipmaps and render with anisotropic filtering when we tilt the map, and render without mipmaps and anisotropic filtering otherwise.