Fix no color values on bloom texture (#13197)

Align meaning of 'exposure' variable across different stages
Put 'exposure' variable behind ENABLE_AUTO_EXPOSURE

client/shaders/extract_bloom/opengl_vertex.glsl

@@ -1,18 +1,25 @@
+#ifdef ENABLE_AUTO_EXPOSURE
 #define exposureMap texture1
 
 uniform sampler2D exposureMap;
 
+varying float exposure;
+#endif
+
 #ifdef GL_ES
 varying mediump vec2 varTexCoord;
 #else
 centroid varying vec2 varTexCoord;
 #endif
 
-varying float exposure;
 
 void main(void)
 {
+#ifdef ENABLE_AUTO_EXPOSURE
+	// value in the texture is on a logarithtmic scale
 	exposure = texture2D(exposureMap, vec2(0.5)).r;
+	exposure = pow(2., exposure);
+#endif
 
 	varTexCoord.st = inTexCoord0.st;
 	gl_Position = inVertexPosition;