Improve mouse cursor location

[TokisanGames]

Fixes #121
Fixes #311
Fixes #225
Fixes #6

Features

• Uses the GPU to detect the location of the mouse on the terrain. Like the previous method, it works without collision. Unlike the previous method, it is instant and has virtually no distance limit (camera far clip hard coded at 100k).
• Works with both the Forward+ and Mobile renderers.
• Provides a render_mouse_layer option to specify the render layer used so get_intersection does not pick up non-terrain objects.
• Ignores holes, so drawing on the opposite side of the terrain is no longer an issue.
• Adds support for Orthogonal cameras
• Adds support for Half resolution viewports
• Adds support for all editor cameras and 4-way terrain editing
• Improvements to decals
• material.get/set_shader_params now provides access to private shader variables.
• Improved usage with vertex_spacing
• Tons of documentation updates