Disabled signed/unsigned warnings like for GCC/Clang
(warning C4018: '>=': signed/unsigned mismatch).
Fixes the following MSVC 2017 warnings:
```
core\image.cpp(999): warning C4804: '>': unsafe use of type 'bool' in operation
core\io\compression.cpp(178): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?)
editor\doc\doc_dump.cpp(226): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?)
scene/resources/material.h(289): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?)
scene/resources/material.h(298): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?)
editor\editor_themes.cpp(379): warning C4805: '==': unsafe mix of type 'int' and type 'bool' in operation
```
This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.
This average is not a proper approximation of a grayscale value,
get_v() is better suited for that.
If we want a real to_grayscale() conversion, it's somewhat more
involved: https://en.wikipedia.org/wiki/Grayscale
Remove the deprecated Gray() from C# bindings as it conflicts
with new named color constants.
Fixes reported logically dead codes by Coverity
* image.cpp: Doesn't really need any modification. But to remove the bug
report then we have to move the MAX call away from the for loop
statement.
* rasterizer_gles3.cpp: Removes unnecessary elif condition since it is
checked earlier in the function
* collada.cpp: If stamement never reached due to macro ERR_CONTINUE does
the same.
* navigation_mesh.cpp: Variables should always be null - however, also
checked for the very same condition in their function call. Leaving this
for review (whether the function call is necessary or not)
* path_editor_plugin.cpp: If cancel is true, then it should restore the
edited value to the original provided.
http://docs.godotengine.org/en/3.0/classes/class_editorspatialgizmo.html#class-editorspatialgizmo-commit-handle
* spatial_editor_gizmos.cpp: the very condition of i >= 3 is
predetermined in the if case right before it. Thus case 1 is always '1'
and case 2 is always '-1'
* grid_map_editor.cpp: Same as above in spatial_editor_gizmos.cpp
* voxel_light_baker.cpp: Same as above in spatial_editor_gizmos.cpp
* visual_server.cpp: Same as above in spatial_editor_gizmos.cpp
* visual_script_expression.cpp: char '-' is already true in the switch
case mechanism. Thus it can never reach to default case.
* particles.cpp: Case 'PARAM_MAX' is unreachable due to index checking
right before the switch execution.
* shader_language.cpp: Invalid index is handled in switch default case.
`type < TYPE_FLOAT && type > TYPE_VEC4` -> `(type < TYPE_FLOAT || type > TYPE_VEC4`)
Fixes the "always false problem" in TODO comment.
Using `misc/scripts/fix_headers.py` on all Godot files.
Some missing header guards were added, and the header inclusion order
was fixed in the Bullet module.
