Add some more graphics feature and limit queries (resolves issue #1234).

love.graphics.getSupported():
 - Added “fullnpot” field, is true everywhere except on some older mobile devices. If false, mipmapping and wrap modes other than “clamp” can’t be used on non-power-of-two sized textures.
 - Added “pixelshaderhighp” field, is true everywhere except some older mobile devices. If false, “highp” precision can’t be used in pixel shaders and love will default to “mediump” instead.

love.graphics.getSystemLimits():
- Added “anisotropy” field, gets the maximum amount of anisotropy you can specify in Texture:setFilter. love will clamp anisotropy arguments greater than this value.

--HG--
branch : minor

src/modules/graphics/opengl/Graphics.cpp

@@ -2009,6 +2009,8 @@ double Graphics::getSystemLimit(SystemLimit limittype) const
 		return (double) gl.getMaxRenderTargets();
 	case Graphics::LIMIT_CANVAS_MSAA:
 		return (double) gl.getMaxRenderbufferSamples();
+	case Graphics::LIMIT_ANISOTROPY:
+		return (double) gl.getMaxAnisotropy();
 	default:
 		return 0.0;
 	}
@@ -2024,6 +2026,10 @@ bool Graphics::isSupported(Feature feature) const
 		return gl.isClampZeroTextureWrapSupported();
 	case FEATURE_LIGHTEN:
 		return GLAD_VERSION_1_4 || GLAD_ES_VERSION_3_0 || GLAD_EXT_blend_minmax;
+	case FEATURE_FULL_NPOT:
+		return GLAD_VERSION_2_0 || GLAD_ES_VERSION_3_0 || GLAD_OES_texture_npot;
+	case FEATURE_PIXEL_SHADER_HIGHP:
+		return gl.isPixelShaderHighpSupported();
 	default:
 		return false;
 	}