Always sample the heightmap with linear filtering in BaseMaterial3D

Nearest-neighbor filtering of the heightmap results in a broken
appearance, with and without Deep Parallax enabled on the material.

Linear filtering results in a more expected appearance. This does
not affect other texture maps such as albedo, normal or roughness.

doc/classes/BaseMaterial3D.xml

@@ -359,6 +359,7 @@
 		</member>
 		<member name="texture_filter" type="int" setter="set_texture_filter" getter="get_texture_filter" enum="BaseMaterial3D.TextureFilter" default="3">
 			Filter flags for the texture. See [enum TextureFilter] for options.
+			[b]Note:[/b] [member heightmap_texture] is always sampled with linear filtering, even if nearest-neighbor filtering is selected here. This is to ensure the heightmap effect looks as intended. If you need sharper height transitions between pixels, resize the heightmap texture in an image editor with nearest-neighbor filtering.
 		</member>
 		<member name="texture_repeat" type="bool" setter="set_flag" getter="get_flag" default="true">
 			Repeat flags for the texture. See [enum TextureFilter] for options.