Fix "incomplete framebuffer" error in gfx_init_texture_rgb(a)

This fixes "incomplete framebuffer" error in Haru Urare right after the
game start, due to gfx_set_framebuffer() being called with a zero width
texture. It was a regression introduced by commit 04d5237, which started
using gfx_set_framebuffer() in gfx_init_texture_rgba().

Ideally, we should avoid creating zero-area textures in upper layers
(commit 3f18b18 was an attempt to do so), but this is difficult because
there are many places that create textures. Let's just make sure that we
don't generate unrecoverable errors.

src/video.c

@@ -569,6 +569,8 @@ void gfx_init_texture_rgba(struct texture *t, int w, int h, SDL_Color color)
 		h = max_texture_size;
 	}
 	gfx_init_texture_blank(t, w, h);
+	if (w <= 0 || h <= 0)
+		return;
 	GLuint fbo = gfx_set_framebuffer(GL_DRAW_FRAMEBUFFER, t, 0, 0, w, h);
 	glClearColor(color.r / 255.f, color.g / 255.f, color.b / 255.f, color.a / 255.f);
 	glClear(GL_COLOR_BUFFER_BIT);
@@ -587,6 +589,8 @@ void gfx_init_texture_rgb(struct texture *t, int w, int h, SDL_Color color)
 	}
 	init_texture(t, w, h);
 	glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, w, h, 0, GL_RGB, GL_UNSIGNED_BYTE, NULL);
+	if (w <= 0 || h <= 0)
+		return;
 	GLuint fbo = gfx_set_framebuffer(GL_DRAW_FRAMEBUFFER, t, 0, 0, w, h);
 	glClearColor(color.r / 255.f, color.g / 255.f, color.b / 255.f, 255.f);
 	glClear(GL_COLOR_BUFFER_BIT);