Particle WebGL resolution should be increased

[jonathanolson]

The particles look quite pixelated on my screen, and increasing the resolution should fix it. #45 should be done first, since increasing the texture size may have negative performance implications.

An example particle on my screen:

[jonathanolson]

Also, if it lowers the performance too much, please revert to the original resolution.

For the code review: #29.

[jonathanolson]

Additionally, instead of just using gl.LINEAR, we may want mip-maps with a larger texture so it looks good across resolutions.

[AshrafSharf]

This issue is related to #45 where a suggestion is made to use a single texture (with opacity value given as a shader input). The issue #45 is now closed after an internal discussion.
Now that we are generating different textures based on the scale factor (sort of internally creating MipMap), leaving the gl.LINEAR option as it is. @jonathanolson please advice if it is fine

[jbphet]

This was addressed when working on #65, closing.