Truncate first segment in TextureProgress 9slices

Much in the way that TextureProgress was truncating the last segment
of the nine slice when the value wasn't close to the end, perform
similar work on the beginning segment. This fixes the beginning segment
always being drawn, sometimes in the incorrect location.

scene/gui/texture_progress.cpp

@@ -251,12 +251,14 @@ void TextureProgress::draw_nine_patch_stretched(const Ref<Texture> &p_texture, F
 
 		middle_section_size *= MIN(1.0, (MAX(0.0, width_filled - first_section_size) / MAX(1.0, width_total - first_section_size - last_section_size)));
 		last_section_size = MAX(0.0, last_section_size - (width_total - width_filled));
+		first_section_size = MIN(first_section_size, width_filled);
 		width_texture = MIN(width_texture, first_section_size + middle_section_size + last_section_size);
 
 		switch (mode) {
 			case FILL_LEFT_TO_RIGHT: {
 				src_rect.size.x = width_texture;
 				dst_rect.size.x = width_filled;
+				topleft.x = first_section_size;
 				bottomright.x = last_section_size;
 			} break;
 			case FILL_RIGHT_TO_LEFT: {
@@ -265,11 +267,13 @@ void TextureProgress::draw_nine_patch_stretched(const Ref<Texture> &p_texture, F
 				dst_rect.position.x += width_total - width_filled;
 				dst_rect.size.x = width_filled;
 				topleft.x = last_section_size;
+				bottomright.x = first_section_size;
 			} break;
 			case FILL_TOP_TO_BOTTOM: {
 				src_rect.size.y = width_texture;
 				dst_rect.size.y = width_filled;
 				bottomright.y = last_section_size;
+				topleft.y = first_section_size;
 			} break;
 			case FILL_BOTTOM_TO_TOP: {
 				src_rect.position.y += src_rect.size.y - width_texture;
@@ -277,6 +281,7 @@ void TextureProgress::draw_nine_patch_stretched(const Ref<Texture> &p_texture, F
 				dst_rect.position.y += width_total - width_filled;
 				dst_rect.size.y = width_filled;
 				topleft.y = last_section_size;
+				bottomright.y = first_section_size;
 			} break;
 			case FILL_BILINEAR_LEFT_AND_RIGHT: {
 				// TODO: Implement